At end-Game this build is very op. The shrooms deals tons of damage and also your spells.
The combo with this build is:
1st Death fire Grasp, this will deal 15% and your next spells will damage them for 20% more
2nd Blade of the ruin king, this will heal you and deal 15% of the health
3rd Blinding Dart, this will deal tons of damage due to the build and activates Lich Bane
4th Basic attack, this will deal tons of damage due to your AD , AP and lich Bane
5th Move Quick, this will activate your Lich Bane and make you to follow people faster
6th follow your enemies with Blinding Dart that will also slow them with Rylai
Tip: You can use a Mushrooms to activate Lich Bane
Tip2: Blind always the AD CARRY this will prevent him from any basic attack damage. Also Blind a enemy if its hitting a teamate.
Tip3: When laning, look at you minions HP so when they are going to last hit you blind the enemy so he doesnt get the money.
Remember: Your Blinding Dart affects spells like: Nasus's Q, Rengar's Q, ect.
Tip4: When laning try to harass your enemies as much as you can but dont push the lane, just last hit minions.
Tip5: place mushrooms in early game so the enemies will stay away from the bushes, then start putting mushrooms outside the bushes so they will be hitten anyways.

The placing of Mushrooms is very easy, you have to put them in the most common places like bushes where more enemies passes. You can learn how to place mushrooms by playing costumes gameand right clicking places in the map so you can put shrooms in the line made by the movement. The best places to put mushrooms securly are common bushes and thin jungle places where mushrooms fit perdectly

You should really try to avoid getting into team fights unless you are very far ahead because as stated above, Teemo is weaker than most characters in team fights and you will often not have the time to place mushrooms properly before a fight breaks out. Try to stick to pushing your lane in for the entire game, forcing the enemy team to send people up which will in turn leave openings for your team to get turrets, Dragon, and steal buffs.

In the event that you have grouped up with your team however, remain in the back along with your AD carry, set down mushrooms to impede advancing bruisers, and blind them so that your AD carry can survive longer. Going out and trying to duel the enemy AD carry yourself will only result in you being focused down instantly.

Despite being squishy, Teemo can take buffs very quickly because of his very high sustained damage. You can take an enemy buff in a very short duration of time, using Noxious Trap (R) to give you vision in the area as you take the buff, then make a quick escape with Move Quick (W).

PROS

Insane damage from levels 1~5 in lane
High mobility
Mushrooms can act as wards\damage\slow
Has a blind
CONS

Short attack range
very weak outside of laning phase
Does almost nothing in team fights - you want to avoid them and just split push entire game