Singed Guide

Tips for Playing Against Singed

-Approach Velocity is actually a pretty nice rune here, as it helps you catch him after hitting Q.
-Make sure you walk your minions into lane against Singed, as he will likely proxy you if you don't (and you'll probably at least get his ghost if you chase him (probably go around his poison though, if you can). Perhaps you can even land a kill if you took Ghost and Approach Velocity.
-Heal up a lot from Doran's Shield, Second Wind, and your E. If he is spamming gas and trying to flip you a lot, he'll run out of mana early on before he can kill you. If he's out of mana, try not to let him recall (you can run up to his tower and then W onto your wave to make him think you're psychotic and diving him).
-Freezing is harder in S25, but if you can it makes it harder for him to run past your tower and proxy, and if he tries breaking the freeze you can maybe spit him into your tower. When he has ult this is the easiest way to kill him.
-If he flips you and his W is down, try to Q + W him. If you land that after first back, and aren't behind, you probably have a kill.

Heartsteel

Singed isn't incredibly beefy, so Heartsteel can feel like some nice damage vs him, plus the extra tongue range and speed is great for chasing and escaping him. Aside from all that, his fling is the only thing that does %HP damage, and you can usually sustain enough health and damage him enough to survive, unless he flips you for free a bunch.
Swiftness Boots

For escaping/chasing!
Hollow Radiance

Hollow is nice for the extra waveclear. I think you can survive just fine off Heartsteel if you're not behind, but you won't clear waves as fast, so determine your role in the game!