Hello and welcome to my very first Mobafire Guide.

For Runes I take:
3x Greater Quintessence of Strength, those will increase your damage by alot and makes your farming alot easier, you are also able to win some 1on1's with these Quintessences.
9x Greater Glyph of Replenishment to semi-spam your spells on laine, those will give you enough Mana-Regen do constantly have enough Mana after holding of a Gank or Engage fights.
9x Greater Seal of Resilience: This is kind of simple to explain, most of your Top-Lain enemys are gonna be based on AD so those runes will lower their damage output on you.
9x Greater Mark of Desolation are for the minimal amount of AD that can be most likely be swapped with Crit or Armor Marks if you want to.

I always thought Manamune was kind of a useless item, but it is really great on Jayce, your Autoattacks will simply increase your Mana, and with your Mana increase your AD increases too. It can help you press the maximum Amount of AD and Mana out of Jayce.

Pros:
- Good Ranged Harrass
- Good Burst Damage
- Semi-Good CC
- Good AOE Damage

Cons:
- Very squishy compared to most of your lain opponents
- High Mana Costs
- OOM (Out of Mana) alot
- Catched in a bad spot, very easy to kill

Never forgett, farming is the key for a good late and Midgame. Try to concentrate on farming alot when playing Jayce.
My farm tips:
At about lvl 7 hit the last 3 Caster Minions with auto attacks and use your To The Skies/Shock Blast to kill them at the same time.
When pushed back to tower, simply use your Hyper Charge to clear the minons easy.

This was my first guide, keep that in mind, i will accept positive and negative feedback.
Enjoy playing LoL.