As above, the key is in preparation.
Strip everything out of it, and I mean everything. Anything that will come apart and can be sprayed separatly should be sprayed separatly.
Get all the major dust out with a vacuum nozzle
rub down with 400 - 600 grit paper (not that critical, the idea is to give it a "key" without sanding too rough that you can see the score marks)
give it another going over with the vacuum.
If you can, wash it in lightly saopy water then rinse in cold (hosepipe works well.
Mask any areas you do not want to spray. Use the blue low tack tape. plug any holes you don't want paint getting into. Tape over the whole of the rear panel (unless you want to paint this also. Look at each panel really up close as sometimes even the smallest mist of paint can get through a hole and settle somewhere you don't want it too, like on the outside of your case. Actually a good idea to use paper and tape to cover the outside of your case.
If you can't wash it then use a tack rag (Halfords where they keep all the sand papers etc) about 99p I think. It's very iportant you get all the dust out.
wipe over with white spirit as you also want any grease from your grubby mits removing.
Shake cans for at least 2 mins, I usually go for 3-4 as the better mixed the paint is the better the finish you get.
Position case on a surface that makes it easy to move around. I put mine on a bit of wood a little bigger than the side of the case. the reason for this is that to get into all the nooks and crannies your going to need to move the case and rotate it. It's easier to get a good finish if you onl;y spray horizontal surfaces. Takes longer but worth it.
Apply grey primer. one light coat to begin with, rotaing case then spraying again. etc until all surfaces have a light coat.
let it dry (2 hours should be fine) then do same again.
The above prep and spraying should pretty much take most of a day. If you've done it in less than 6 hours you've rushed it and it won't look good.
Let it dry over night.
Check all your masking as it can come away.
If you have any imperfections you have 2 choices.
sand them away lightly (tack cloth to remove dust)
Leave them if they're not too bad or in a place that won't be seen.
Spray top coat as per primer, light coats, rotating and spraying as you go.
Watch the paint in the corners and the hard to get at places as the temptation is to give these a "good squirt". resist the urge as you wuill almost certainly get a run.