So I've really started to enjoy playing Jayce, but frankly have no grasp on the correct order to use his abilities. I have his e q combo in gun stance down. But what's the proper mechanics for switching to hammer and such? FYI I usually play him top lane.
Jayce is pretty flexible. The standard cannon to hammer combo is (starting in cannon) q/e, wr, w/q, land the auto attacks and e. You can do the hypercharged autos before or after hammer form, doesn't really matter. If your matchup is melee and can't gap close you can often start in hammer form, qwe and then turn to range form to finish off.
You probably already know pressing e after q in cannon form gives the opponent less time to react, but if you can't land them accurately it's better to lay the e down and take aim. Also put the acceleration gate as close to you as you can (you can put it on top of yourself and still have your q accelerate through). This gives you the speed boost right off the bat. This also helps in case you realize you don't want to go in (say you see the jungler). If you put your acceleration gate in front of you for your q/e then you wouldn't have gotten the speed boost. This mechanic can be a little tricky at first and isn't worth doing if it affects your aim.
I would watch some high elo/pro games and watch what order they use their skills. From what I've seen, I think hammer stance is usually AoE-thing > gap closer > knock back (which, I think, is WQE?). That's a pretty common combo, where you activate your W either before you jump or while you're mid-air (I think you can do that), but when you're really good at Jayce (as with any champion) you want to not just do these combos as muscle-memory and actually think about what you want to accomplish (for example, hold your E until you know you want to knock them back, or, knock them back first, esp. in a direction you want, then jump on them).
Try activating Hyper Charge after an E-Q attack, switching to hammer mode and diving the target with Q. Autoattack 3 times (because Hyper Charge for some reason can work on Hammer Form) and then E your target. It's a fun all-in or trade combo that really minces a target.
