Erm.. Gonna have to cast my mind back a bit..
Basically the game allocates certain positions before others. So the best fielder at mid on will go to mid on first. I think it's mid on/mid off first, then other infield positions, outfield (inc midwicket), slips and then close fielders (inc gully). When there are players left they go to slips or gully (why bowlers often ended up there because they might have low infield ratings). To be more realistic I would try and have two bowlers set at Mid On/Mid Off which is fairly realistic, then another two as outfield. You want your slip fielders to be low rated for other positions - sucks for the likes of Ben Stokes and Steve Smith who can field anywhere - particularly if you go into a limited overs match.
Best way I find to do it is have a 1st choice and 2nd choice position and rate them highly and rate the others low.
I did have an order noted down in a spreadsheet but I've no idea where that is and I may well have deleted it. I'll see if I can find it.