Your goal with your runes is to compensate for Teemo’s limited built in durability. Most of it is standard fare. The only thing that might need explanation is armor per level Seals and flat magic resistance Glyphs. My reasoning for that is, magic damage is most dangerous to Teemo in the early game, when it hits like a truck. Once you’ve got your Banshee’s Veil (And possibly your Mercury Treads and Abyssal Scepter), you will have no problem surviving their pitiful burst. On the other hand, there are a lot more options for mitigating armor later in the game, making higher end game armor a bigger priority.
One weakness of Tank Teemo is his low built in Health. while it is tempting to grab Warmogs to compensate for that, it is far too expensive for what it gives. Instead I focus on grabbing items that give both damage mitigation and health.
Doran Blade (Ease of last hitting, added survivability, makes Teemo a better early game ganker, sell to complete other items later)
Boots: Mercury Treads (Enemy team is primarily magic or CC heavy), Ninja Tabi (Enemy team is primarily physical)
Sunfire Cape (Get this before Banshee’s Veil if enemy team is physical)
Banshee’s Veil (Get this before Sunfire Cape if enemy team is magic or CC heavy)
Post Core Items:
Aegis of Legion (Enemy Team has evenly distributed damage)
Abyssal Scepter (Enemy Team has heavily magic damage, or is not hurting you at all, Abyssal Scepter boosts damage from your poison as well as Sunfire Cape)
Guardian Angel (Enemy Team is heavily physical)
Frozen Heart (Alternative to Guardian Angel, CDR is nice since the build has little otherwise)
Initiation: Your role as the tank is two fold: soaking up damage and initiating. To accomplish these goals, you will do what most people love to see the opponent’s Teemo do. You will charge head first into the crowd of enemies with your team shortly behind you. Due to your amazing Teemo Taunt you will instantly receive focus from the entire team. Once you’re there in the middle, blind the enemy AD carry and start attacking. Unlike other tanks, your Teemo Taunt will not wear off, and that carry will stay and fight no matter how ineffective his/her damage is.
Aggressive Shroom: Your average Teemo has to preemptively place Mushrooms all over the map and hope the enemy walks into them. Not you. Tank Teemo has special Mushrooms that he can run up and drop right in front of the enemy. He doesn’t fear the retaliator fire he’ll receive. This does mean you should have at least 1-2 Mushrooms ready between fights.
Bait: Wander about ridiculously deep in enemy territory while your team hides in a bush. Your Teemo Taunt will draw the enemy to you. Play squishy Teemo and run like hell… right into your team’s bush.
There are many more strategies that can be accomplished with Tank Teemo, but it’s best if you figure them out for yourself. I will leave you with one last note though: People will tell you that they check the enemy’s items and if they see you build tank, they won’t focus you. Chances are, they’re lying, not about checking your items, but about not focusing you. The Teemo Taunt is unavoidable.