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I've built PCs from scratch and rebuilt/upgraded my PC on several occasions. (In fact, the hard drive on "Ad Astra" just completely borked out so I get to tinker on her again. Thank god for backups.)
I won't repeat all the other advice you've gotten.
But, I will tell you to do the project bare foot. Keeps you grounded, and if you drop any teeny parts on the floor, and can't find them, you can walk around and if it's there, you will feel it when you step on it.
If you don't have needlenose pliers/forceps you need a pair.
Have fun! You won't necessarily save money, but you will get an excellent computer with no "mystery" parts in it.