You don't actually need any fancy drawing skills to make the Tally Hall men recognizable. They've made sure of it. They are helpfully tagged, labeled, color-coded for your convencience. (Unless you're colorblind, in which case I'm so sorry.) But I happen to like the art of caricature, so I like going that extra mile. Here's some stuff I've noticed.
Don't take any of this too seriously; there's no "one right way" to draw someone unless you're going for photorealism (which I am not). Most of this is based off the T.H.I.S./Marvin's era appearances since that's easiest to find references for.
Note: these are static thoughts from a snippet in time while my art style evolves freely; these will become rapidly outdated with every drawing I do.
Maybe I'll come back and finish Bora's (and perhaps do one for Casey, too). But don't get your hopes up.