There are numerous resources on this and a blueprint for the AR15 lower...
Basically you need to deck the upper by milling it flat with a very large end mill
Then you mill the magazine well. People have at first said you have to use a EDM or a long broach but they have been done with a CNC mill and a lot of patience as well as files.
You will have to then drill the fire control holes, mill out the fire control pocket, not too hard to do, exactly same as doing a 80%.
You need to drill the takedown/pivot holes and that requires some precision, make sure your mill have DRO to facilitate this (in most cases you will drill those holes right after decking the lower to give you a basis to index from)
You will also need to drill, ream, and tap for the buffer tube, as well as drill for the retaining pin (they are angled at 11 degrees)
then the rest like trigger guard slot, hole, etc.
It's not really that hard honestly as long as you got the right steps or CNC programs but unless you already have pallets of the forging for next to nothing you could in some cases buy 80% forged receivers from Tactical Machining for 39 dollars, and 0% forgings are selling for around 25 each if you are lucky. Do it for the learning experience.
Right now I'm milling a Remington 700 action from a bar stock, I wish there are 80% versions out there but there are not other than the broached blanks selling for over 200 dollars. Some creative ways was needed to handle the broaching job without a EDM or broach.