Otto-
Each manufacturer is a little different. Some have their policies posted on their website. Pick your prototype, find out who makes it, talk to them, get the template for the type of equipment, do your artwork (or have them do it - in my case it was simple since IM does a bunch of WM equipment already - they did the artwork very quickly), send it to them for an estimate, pay 50%, sign agreement, think to yourself "what have I just committed to!", wait what seems like an eternity, then wait for Chinese New Year's month long delays, then let the MFR do their QC, pay they other 50%, get your product. Along the way you also need to get the word out that you are doing the run and hopefully sell enough of them to make your money back.
For IM, the minimum engine run was 100 pieces (50 of each road number, could have broken it down a little more - but the WM only had 51 & 52).
For rolling stock (boxcars, etc) the minimum run numbers are higher (around 300-500 pcs, but you can have multiple road numbers). Depending on MFR and when they are running the specific type of product - you can expect the process to take at least 6 months, sometimes years... I got lucky and caught IM just as they were doing the F3s. I contacted them at the end of June last year...
-alex-
