Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build only works effectively if you know the game will last a LONG time.
Singed is the kind of champ that works well only if the rest of his team helps him with EVERYTHING, if they don't let him do what he wants then they're stuck with a bad tank that feeds because the other team specifically targets him.
But he is also one of the best carriers of the game, scaring away enemy champs with his mere presence (If you show off your Fling in early game well enough) and dominating the lane that he's in. This Build tries to meet in the middle ground between the two, it works its best when the rest of his team helps, but he can solo most things with this build.
Reason for spells is because if you've played him long enough you'll realize that no one wants to get Fling'd so the enemy team will go out of their way to take you out with everything they have. You can counter this with the Guaranteed Cleanse + Ghost getaway if things get too tough or your team isn't helping to scare them back.
Get Fling instantly so you can scare some cocky enemy champs into shape and show them who's boss, and for very Early game ganks (hide in the bushes and wait for the late guys to peek around so you can start bringing the pain. REMEMBER to NOT fling them to your allies until they start running away so you can start off with a well coordinated gank and make the enemy team afraid of you already.
Now for items; instead of getting ROA first, go for Teardrop which you can turn into Archangels later in the game and then start farming for a Warmog's so you can get increased health regen for Mid game so you won't have to worry about buying Health pots.
Now you should have Warmog's, teardrop and Boots of Swiftness, but if you don't, then start farming IMMEDIATELY so you can get them start powering up Warmog's for late game as they will impact it a lot.
By this time you should be doing a few ganks here and there with some surprise mega adhesive and fling so you can start gathering gold for your ROA's.
If there are a lot of hardhitters on their team and they like to target you, note the other team's damage output and get either a Thornmail or Banshee's Veil or something in between before you get the ROA's. Also remember to push as hard as you can to make the Late game as easy as possible.
By now you should have plenty of assists (and kills if you're lucky enough) so your Warmog's will be completely fed and you'll regen health like CRAZY which means you can start backdooring turrets while your team distracts and when they come to check up on you sneak off and start hammering at another turret. If you can get away before they take notice you can surprise their squishier ones with a well timed fling and get some quick gold and spook the team a bit before going back to hammering away at their turrets.
If you do everything right (Which basically sums down to NOT feeding) then you pretty much have a guaranteed win.
I explain which items in the build above, BUUUUT most people will flame about why I'd choose Teardrop and eventually Archangel's over a good defensive item, and I will answer it bluntly; it kills two birds with one very conveniently priced stone.
Trust me on this, the mana boost from Teardrop helps if you leak (Use Poison trail excessively) early game and his innate converts mana into bonus health, and when you turn it into Arch's you get ability power + increased mana regen. I know that Tear doesn't work with Poison Trail, but you should still be using your abilities like crazy regardless to pump it up anyway.