First start off with Mercury Canon, and transform to Mercury Hammer afterwards. The idea is, that when you transform from Canon to Hammer, your next basic attack will deal an extra amount of damage. Use your Q to start harassing the enemy, then basic attack once and then pop your E to finish ur burst. That's pretty much the idea of how to play Jayce in a good and safe way, which will keep your enemy(ies) from farming. And the technique works especially well against squishy enemies. Good luck and have fun.