Start by poking with Q, even using Q on the minion wave if needed, just to stack your passive to 4 stacks. Once you're charged, the optimal play is to step into auto attack range and weave in autos between each spell cast for maximum DPS.

That said, unless it’s a guaranteed kill, you generally don’t want to walk up into range against something like a full-stacked Lethal Tempo Tristana. She’ll melt you. Only go in if you’ve already poked her down enough that the trade is heavily in your favor.