This is a very important time for every Teemo player. At this point you will decide how annoying you are going to be in the next stage of the game (team fighting). If you find you are against a very easy lane opponent(ex, Garen/Tryndamere) you will need to snowball very hard on them in order to get the three core items of your build that make you annoying.

If you manage to get Mallet/BORK/Gargoyle you are ready to become the annoying monster you have always wanted to be.

This is important. Keep in mind they are not placed to burst people with this build. they are placed with the sole intention of making people miserable when they step on it and get slowed into oblivion and caught out.
these are some shroom placements I like. commonly jungle entrances are a good spot which I have marked on this map. You should remember that you can also place these on objectives (Drag/Baron) for vision when the enemy is trying to take the objective.

Team Fighting
At this point in the game, you are going to be grouping with your team and helping getting objectives and kill the enemy team. The job for you with this build it to stand in the front of you're team and tank everything well at the same time auto attacking high priority targets to slow them down and make them easy targets for assassins. on the other hand, you can also focus the enemy front line and keep them slowed with BORK active and Frozen Mallet so that they never even get a chance to touch the ADC in the backline on the fight. Also, remember you should have locket at this point if an enemy assassin or tank manages to get past you're annoying as hell slows you can pop gargoyles and locket to give the carries a FAT shield of 1k health or more.

Pros

Tanky
Big Shield
Annoying
Blind for ADC
Peel
Good sustain

Cons

No CC
Pretty much outdone by any actual tank
Low damage output