Your line phase is easy, you just need to survive aggro enemies to don't make them a big threat, the best thing to do in early game is just aa + Q + aa to active Phase Rush and be able to escape, is a nice trade too if viable.
Versus passive/bad early champs you can use the combo and phase rush to anihilate the enemies, you can make a good early burst and keep the attack range to keep poking

Use the mushrooms, is the idea of the build, you aren't the damage dealer of the team but you are a threat to them because you can run to every part of the map and keep using mushrooms and since your build is more defensive than the average Teemo build you can go to sites taht the other Teemos can't because they would be deleted.

You are level 16 now, you have 3/4/5 items, is your moment, you can rush whenever you want to every part of the map and force the enemy to keep having a red ward in less important map parts and since you have that movement speed you can cover a big zone in a small time

I love this Teemo build, it haven't got a lot of damage or dps but at least in my gameplay it let me do things that with the other Teemo builds I play I was never able to do.

Run, keep running, you can't die without 2/3 people hardfocusing you for a couple of seconds because you can outrun almost everyone, use a lot of mushrooms, with the new R mana cost and the Crown you can put infinite mushrooms in every spot availible, you can also try to maximize even more CDR but I don't think that is worth enough