Lol, you're reviving all the old case mod logs :P
There are different ways depending on budget. There are others here who know more about painting than I do who will probably post better info but I do 1 of 3 things depending on how much $$$ and quality is desired.
1) For best finishes I have the case sandblasted as prep and sent to a poweder coaters to get done. Costs more but looks beautiful and is very hard paint so is very durable.
2) For cheaper DIY I sand it down so the primer will bind better, then use a self etching primer, 3 coats at least...many thin coats are better than a couple thick ones. Then I visit my local automotive paint supply place and get the colour I want (image below, 3 cans on the right, catalyst, hardener and colour). This is more costly than spray bottles but usually cheaper than getting it powder coated...better quality than spray cans, the finish will by like a car finish if done properly.
3) For a straight forward flat black look the automotive place should have some spray cans of flat black, here is what I use for most builds.
See the two spray bottles on the left, primer and paint. Spend a few extra bucks for the good stuff and you won't regret it...will hold up better in the long run.