I'd like to say I have a plan, but usually it turns into an "anywhichway ya can" affair. You can plan all you want, but until gun is in hand and has been measured, you don't know for sure what you have to work with. Sometimes I buy a gun/barrel/reamer to fit a mould I already have, but usually it goes something like this......
After I clean the thing and make an impact impression of the chamber and throat (or whatever is necessary, as in measuring a revolver's cylinder), I sigh real big in disgust and lean it in a corner for a few days while I mull over how I'm going to deal with the almost invariable dimensional paradoxes. Part of the subconscious number-crunching and mental 3D analysis involves figuring out if I have any moulds already that will fit and work for the intended purpose, or if I'm going to have to spend some more money. After sleeping on it for a while, if I have something close, I make some dummy rounds and check them with a sharpie, playing with seating depth and sizing to find out if I got something good enough to try. Then I build some loads and go shoot. Sometimes it works, sometimes it doesn't. If it does, great. If it doesn't, then it's on to the long process of figuring out why, fixing the gun, trying different loads, bullets, alloy, etc., which happens to be a large part of my entire time spent in the cast bullet hobby. Sometimes the throat has burrs and needs to have the bejeezus shot out of it with whatever random jacketed bullets I have laying around, or maybe a bunch of dry paper-patched bullets, or throating may be in order, or more severe firelapping....the list goes on. If I don't already have a mould on hand that will work, I dig around the usual catalogs and see if something already exists that will work, or if I need to have a custom mold made.