In making a custom loop, half the fun is in experimenting and making it look pretty.  Sometimes I kind of just look at mine for a minute or two even thoughts not the baddest build around I'm still happy :)
 
You might want to decide on a case first.  Lian Li O11D XL is a very common choice.  I did a Fractal design Define 7.  To get ideas on what you can do with the case you pick out, punch that case name into a youtube search with "custom loop" appended and see what you can find.  Then from there, you can see a handful of custom and complete systems and make your choices on parts from there.
 
At minimum, you need a few radiators, a GPU block, a CPU block and a pump.  Might as well get a pump/reservoir combo.  Then you need tubing and fittings.
 
There are two types of tubing... hard and soft.  And then there are different dimensions of tubing diameters.  Make sure you get tubing that fits the connectors you like.  I did 10/16mm EK ZMT rubber tubing.  its very easy to work with, fittings are usually in stock at US based stores like https://www.performance-pcs.com/ (no affiliation).
 
Dont mix aluminum fittings in with copper/nickel/brass.  Stick to copper/nickel/brass.  It does matter.. fluid.  If you go with distilled water, you might want to get some Hades Extreme and put a couple of drops in your loop res every month or so.  
 
 
^^^ Got speed nailed it
 

Asterothe1905,
Since you already have the hydro copper gpu, it makes things a lot easier as now you wont have to disassemble your gpu to custom loop your pc.  Redteamtango covered a lot of the subject matter so I'll just go a little further to try and help you.  
 
Case is very important as it provides you the foundation for your build.  If performance is your goal, then you want to make sure you have enough radiator capacity and room to work with for your performance targets.  Budget also has a huge impact as well so if you are limited in budget then you also want to do a ton of homework to get the most performance out of your monies available for your build.  Depending on your ambient temps and what you do with your pc, you will need to take those into consideration.  If your room is climate controlled or very cool then that will help lower temps however if you live in a warmer environment and don't use a/c then that will be a larger factor as heat eventually affects performance.  The Lian Li O11XL is a common case for builds.  It is a pretty large case so be sure to look at any manufacturer's website for case dimensions as well as for radiator support for cases you might be interested in.  Plan for a larger build to support enough rad space and so that you are able to work in.  Smaller cases can be more challenging to install enough radiator capacity to support loops so unless there is an absolute need for a smaller case then I'd stick to something like the O11 XL.  There are tons of videos on youtube of people using that case so it can give you ideas on what parts to use and installation of those parts into the case.  
 
I know from the past Redteamtango started out with a smaller case and he moved his loop and components to a larger case after he didn't get the performance from his custom loop he was seeking.  Limitations on radiator capacity affected his loop's ability to cool his components which lead him to change case and add radiator capacity to gain the performance he was looking for.  He can detail more of his experience on this matter.  
 
The open loop should be planned before anything is bought.  Think about the radiator length and thicknesses you plan on using and their impacts on cooling.  Redteamtango also covered the types of tubing you can use.  For first time users it is much easier to use soft tubing to route your coolant through the loop.  Be sure to research fittings and purchase tubing to match the diameter of the fitting you choose.  Some fittings just slide the tube onto it, others like EK not only slide on the fitting but also capture it as well giving you better seal and less chance of if slipping off.  Pump/res combo are nice, but getting them into your loop can be tricky at times.  Companies make distroplates which mount in the case that have ports you can use to route your coolant.  If you go with soft tubing the benefits of the distroplate are diminished as the soft tubing is fairly easy to route through the case.  The distroplate with pump is more costly so if you are going with soft tubing then I'd stick with a pump/res combo that you can fit into your case.  I like pump/res combos with D5 pumps for their high output and long life.  Malbadon also recommended the use of a CPU block which is absolutely the right call if you are going to water cool your gpu.  I would recommend the Optimus line of CPU blocks as the cost is very reasonable for the materials and performance.  The main drawback to their product is lead time.  When it says in stock, it really means they have the materials to make your order so if you order the cpu block it may take around 4 weeks to get it in.  But the block is really good and the materials they use are top notch.  
 
Hard tube system are not that difficult to do, but it does take time and patience.  If you are unsure, the safer bet would be to use soft tubes, but I just want to share that using hard tubes doesn't require expert knowledge or knowhow.  It does require to research and practice doing bends and then planning bends and layout.  Anything wrong in the bends or layout are more difficult to overcome with hard tubes.  If you do use hard tubes, that's where the distroplate/pump setup can come in handy.  The distro plates can be used for cases which make the use of hard tubes much easier by reducing the need for tubes to have more than on bend in them.  A tube with a single bend is fairly easy to do, it's when you have tube routing that requires some to have more than one bend where it gets a little tricky for some.  
 
Lastly the components of your pc needs to be considered.  You already are getting a pretty good gpu, so I assume you would want to get a decent overall experience.  Get a Motherboard which is middle of the pack.  You can go high end, but you probably wont use most of the features so it could be considered a waste of money.  Mid level board depend on your CPU choice so I can't recommend anything specifically but for X570 I like the Strix E and for Z690 it would come down to your ability to get DDR5 ram.  For the Z690 if you can't get DDR5 for a decent price I like the Asus Strix A board.  There are a few boards from MSI that are very solid, but the quick release PCI slot is handy, the VRMS on that board are solid, the bios is easy to navigate, it has bios flashback in case you need to reflash or update bios, it has a good amount of nvme ports, built in wifi if needed, and it's a unicorn white board for less than 400 bucks on amazon.  if you can get DDR5 ram then the choices really open up so then you would need to do more research.  I bought the Strix A for my son's pc upgrade that I need to so which is why I researched that board.  For me and what I need for his gaming needs it was the board for the build since he already has a white theme.  Read the manual and make use of the gen 4 nvme options to max out performance.  Also I would not skimp on the PSU.  EVGA has some pretty good deals on the 1300, 1200, and 1000 watt psus.  I would get the 1300 if the budget allows.  The pc will only pull power it needs regardless of how much unused capacity the psu has.  This just gives you more headroom in the future.  I'd also get atleast gold rated for it as well.  
 
Plan your build and make sure to budget and layout all the cost on a spreadsheet so you are able to see what you are spending and if you covered all your needs.  This is the among the first things I do when I help people with their open loop.  Creating a spreadsheet helps you keep your cost within your expectations and allows you to see where the money is going in case you need to pull back some.  Costs for custom pcs can get out of hand and things like fans, adapters, extensions all start to add up.  Especially fans as they can add a lot to your build costs depending on the choice you make.  
 
Hope this is helpful to you.  Good luck with your build and enjoy your Hydro Copper 3080.  It should be a very good gpu.  

Low FPI (Fins Per Inch) Radiators combined with low RPM / High Static Pressure Fans i the key. to keeping your coolant cool....
I've had a coolant temp probe in my WC'ing misc parts box since 2014, have never used it.
 
Once you get intimate with your loop you will know exactly how effective it is by simply monitoring your GPU temperatures (if your GPU in incorporated into your loop).
 
Idle GPU temps in an effective loop should be very very close to room ambient temp, this will be representative of your coolant temp.
Once you game or throw an intensive benchmark at your GPU, the lower the temp on load, the more effective your loop.
 
My current loop for example:
3080Ti w Vector Water Block (480*360*240 Radiators) all Low FPI w High Static Pressure Fans, Fans set n forget 1450 RPM, Pump set n forget 3100 RPM.
Ambient 20c
GPU Idle 20c
GPU full load 42c
After shutting down the Program it takes about 2-3 minutes for my GPU to return to ambient 20c.
The above temps alone let me know that the loop is very effective.
 
Once you close a GPU intensive program the quicker your GPU drops down to (close to ambient..also representative of coolant temp), the more effective your loop is.
 
Order of components and pump speed dont make much difference at in coolant temps, in an effective loop regardless of order or pump speed (obviously not dead slow or full snot), the coolant temp should be very close to ambient, this alone lets you know how well your loop is working.
 
As said at the front end, low FPI Rads and high static pressure fans are key.
 
I have been setting up and running custom loops since 2013, they really are very simple, very robust and very effective.