Make a proposal, ie;

I want to make a mod set during ;date/period (description of period)
Factions; Blah blah.
New map; (screenshots if you have them)
ect ect..

Modelers tend to be rather rare these days so to snare one you need to have something to show, odds are (as this is RTW) the models you want have already been made, you just have to go and find them and seek permission to use them.