Hello,
I already play LoL for some time now. Though I do not play it very competitively, more for fun. I am far from being a high mechanicly Diamond player but I think Singed is a great champ and really fun when playing. Many people think Singed is really easy but don't be fooled. I will show you there is quiet some depth to a rather bland looking champion.

This is the REALLY hard part for Singed. Everyone builds Singed in a different way and many of those are wrong. Singed does not really need much mana to work well, especially when you know how to play him and how to control your mana.

- As stated in the notes. The Dark Seal is not for the stacks, it's for the extra healing on the pots. No, i won't deny having loads of stacks and so loads of AP is fun but don't focus on this.

-Rylais is so good. You run fast, your enemies don't! More explanation needed? I think not.

-The Abyssal Scepter is really nice, especially against heavy AP comps. It also lowers enemy MR and gives some CDR to ult more often.

-Liandry's is not that good in all situations. I put it between the core items because it can be very very good. You build it against almost any heavy tank or mildly tanky enemy team.

-For the armor item you should look at what you really want. Do you want flat out armor and are you against an AA oriented team? Go Thornmail. Are crits a problem, Randiun's is the best choice. I would always think of DMP as the best choice because of the extra MS it gives but the others are deffinitly good too.

-Your choices in MR are rather small because of Singed being a hard tank and not a supportive tank like Bard for example. Banshee's is good for engaging but Spirit Visage is nice in combination with your ult. Your choice.

For boots you can choose between 3.

-Mercury Treads: Very good against an AP toplaner and helps for countering heavy CC. You will feel the difference.

-Boots of Swiftness: Gives no resistances so not the best choice against heavy AD or AP teams but good if they have a lot of slows.

-Ninja Tabi: I like these boots. They really do help against strong AD toplaners like Pantheon. Pick these up along the way to counter a lot of DMG.

So here is the fun part.