Teemo is one of the best harassment supports. He can hit-and-run once he gets 1 level of [W] and upgrading boots first guarantees this advantage for the rest of the game.

You need to be very map aware as a support and Teemo shrooms should be placed in bushes or along choke points. The limited vision granted by shrooms combined with the slow effect when someone steps on them makes them ideal for warding.

Shrooms can be used directly on a wave, but this should be reserved for when you want to target the turret alongside a wave of your minions.

The most important part of Support Teemo is positioning yourself to always be a threat. Teemo can camoflage, but making yourself visible on purpose can have the effect of pushing the enemy too far back to reliably get minion farm or exp. Hiding and popping out contantly makes the enemy start suspecting every bush and keeping back to avoid harassment (and fishing you to chase, it's a trap).

During laning phase, don't go for kills. Just harass and build stacks of dark harvest. Let the carry get the kills if they want.

Prioritize the adc with your Q blind.