I,m sorry I can,t be anymore specific than that. for example lets say I have a chunk of steel and I want to mill it square and i have a milling cutter which is lets say 3/8 diameter, as I start to mill , I,ll move the cutter so its about an 1/8 of an inch over the steel chunk and about 1/16 to 3/32 depth of cut, I,ll make one pass over the steel , come back move the table so I,m another 1/8 over and so on. But it sounds like you'll be milling plastics so you,ll be alright. Believe me when you make a mistake you most likely will not want to make the same mistake again thus lesson learned. Here read this link
http://www.americanmachinetools.com/...ng_machine.htm
How to use a Milling Machine