It’s actually rather simple, or at least the way I went about it. First thing is buy a kit. This will include everything you need but the lower. I got one from PSA for a little over $320. If you are patient you will find they run sales and you can save another $20. Surprisingly it came with a 4150 chrome moly barrel and a forged 7075 T6 receiver. It was sort of a M4 knock off, 16” barrel with 1-7 twist, M4 adjustable stock, pretty much everything you will need. Trigger, buffer tube, pins, everything. Mine even came with Magpul sites.
For the lower I just bought an Anderson at a local pawn shop. They made all of Colt’s LE branded rifles at one point. I’m not sure if they still do. But, it’s a mil-spec part. It set me back $70.
Once you have those items it’s just a matter of sitting down and assembling it. Take your time, no special tools are really needed, but some special tools would speed up the process. Though I have the tools I purposely only used a small punch, a hemostat forcep clamp (for the front detent pin) and of course the buffer wrench.
There are a vast number of YouTube videos that will walk you through the process if you’re a little uncomfortable on your first go. I would say the trigger will freak you out the most. Though once you install it you will laugh at yourself when you realize just how easy the process actually is. Give it a go, it is a fun way to spend an afternoon without breaking the bank. And there is a strange satisfaction firing it for the first time knowing you had a hand in it.