Welcome, to my Signed ranked guide for AP and Tank. This is how i play Signed you don't have to like it but it's my playstyle with signed and i have 80% win change out of 10 games so quite succesfull for me.

As Summoner Spells i take flash and exhaust
Flash i take for every champion since you can escape like a boss with it it's better then ghost... teleport since you want to farm farm farm specially in top lane it's good for teleporting into teamfights.sometimes i take exhaust i take for chasing and shutting down carry's
you can take ghost which is good for chasing...

In teamfights you want to run around crazy with your poison on and protect your carry and use fling by shutting down their carries. and slow by chasing his poison does lots of damage and you get many kills since your AP signed.

As tank you do similair thing but you exhaust the ad carry eg ashe and you make them do nothing like this and fling people with low hp back to your team and you have to initiate with poison trail on ulti on and fling someone in ur team

Farming with Signed is not difficult use your poison run around minions when they get low last hit them keep doing this. when you take teleport you will split push all the time and when a teamfights starts you can teleport to a ward close by so signed is a great pusher easy farm and you will get lots of minion kills.

So as Ap Signed you want to be pushing nearly all the time but play safe and ward around you so you won't die by ganks from their jungler. when a teamfight start you can teleport to a ward and then destroy your enemies. As a tank Signed you farm too but you have to initiate most likely so you need to know when teamfights starts but still you push but when eg 4 people from your team go mid you must go their too because a teamfight is likely to start and you have to initiate.

Pro's
Amazing pusher
Very tanky in begin till end also when you play Ap singed.
awesome ult
one of the best slow's in the game
His fling is just awesome.
Cons
he only has 2 abilities which scale with ap

Signed is a very viable champion to have on your team but needs farm so solo top would be very good with him. i think ap signed is better then tank but in ranked you have people who say all the time "Youre noob you play ap singed he should be played as tank" well ignore that signed is good ap. but again problem in ranked is nobody plays tank in my elo 1300 so that kinda sucks.
As tank signed dont forget buy items depinding on your enemy team if they have Ad much Get for sure a frozen mallet and thornmail frozen mallet is personally for me the best armor item for signed since his passive works well with it you get the health from the mana then so... and if you play against ap get magic resist always adapt to your enemy team