Jayce is a really good champion, i use it as a top laner. He gets tanky at level 5-6 and do alot of damage with this build. It works good for me at least. You need to play safe for the first levels, then be agressive with your combo.

Mercury Cannon : while using the mercury cannon , i like to do this simple combo, Acceleration Gate in front of you, Shock Blast on your target, then, if im close enough i like to use Hyper Charge to do a little more damage. This is really easy to do, and you must do this do harass your enemy correctly early game.

Mercury Hammer : While using mercury hammer, you can do this little combo : Use lighting field, then jump on your enemy with To The Skies! really fast, then use Thundering Blow, run back or keep fighting at this point. If you think your enemy is low enough, keep attacking and do the Mercury Cannon combo and dont forget to run you your Acceleration Gate to get closer of your target.

You can use both of these combo together to do alot of damage but take care, you might get low on mana really fast.
I usually go for the kill if the target is squishy and if my mercury cannon combo deals good damage, otherwise, i farm until giant belt/frozen mallet to really start being annoying.

I start with my E in case i get ganked early game or in a bad situation, starting with Q is useless since you dont have your acceleration barrier to do good damage. The W must be maxed last, but you need to unlock it early game because it gives you a good mana regeneration passive while using the hammer. The Q is good early game as seen in the previous Mercury Cannon combo, but when your enemy get some life and become tanky, your Thundering Blow will be better because it gives damage based on your target life, so if your target have more life, it means more damage .

I use these runes to have some good damage while having some armor, you might use your own set of runes, nothing else to say about that.

I use and , you might also use and to get back to your tower fast if you die top. Use your ignite only if you are really sure to get the kill with it, you can use it if your target is far and low health. But you might also use it on people having a healing effect, just like volibear's passive. If you are figthing volibear, dont forget to use ignite BEFORE his passive starts, so that you can avoid alot of health regenerating.

Early game i like to use mercury cannon to farm, you play safe and farm until you get frozen mallet or earlier if you feel like it. Later during the game (level 3+) i still use mercury cannon, but when i miss more than half of my mana, i use the hammer, because it gives you back your mana.

This is my first guide ever, i hope this helped you to play as a Jayce, im not even sure if somebody will read this, but thanks anyway ! Leave a comment if you have anything to add or to say. Good JAYCING