Basically like they say...
Start like so:
and so on.
Early on you want mana regen and cdr. Mid to late game you start getting AP to make things hurt more.
Grenades cool down is really low, learn to target them, get good at aiming where they're going to dodge to, etiher beside, or behind them usually.
Also remember that grenades can damage towers.
The reason for turret + ulti at 5/6 is that you can go offensive with an instant green turret that way.
Say you're chilling in mid and want to push the tower a bit, you've got a full wave of creeps and are about to hit their tower. You can run up and green turret in tower range so it gets extra hits on the tower, as well as grenade the incoming minion wave so they're blinded, there-by allowing your minions to live longer.
The green turret drop also works if/when you get jumped. Say 2 heroes jump out of the bushes and try to hump you. (usually jax, nidalee, etc). You drop a grenade instantly, blinding and (if you're lucky) stunning them, and then also drop the green turret right away. Then proceed to walk away from your tower a little bit so they chase, but still get hit with your turret. When they're near the edge of range, use flash to go backwards to your turret and head back towards your tower. This will get your turret a ton of hits and cause most gank attempts to fail. Also during that time you can get 1-2 rocket shots off (1 initially either immediately before or immediately after putting up your turret, and then a second one after flashing on your way back past your turret)
Obviously that tactic will work better when your AP is higher for a better dmg turret, but I'm sure you get the idea. (Also won't work without flash, but heimer is slow and imo he needs flash... I've gotten many kills from using it).
If you're unsure on runes, go with some combination of mana regen and cdr.
/edit -- if you don't understand green turret drops, it's just dropping a turret and instantly using upgrade on it. It's the bees' knees.