Tell them politely and don't hurt their feelings. Just like Tymps. said, a tad or a little bit makes it sound much better.

This thread was inspired by my blogs wasn't it?

I wish I knew. There's this one boy I know who does gospel choir at our college and he sings with Beyoncé-style runs at all times in an annoyingly nasal voice. He thinks he's hot **** and that's exactly what he sounds like: a steaming hot pile of doodoo. I cringe every time I have to listen to him; one time he caught me rolling my eyes while he was singing and I had to come up with an excuse.

And don't get me started on his daily 60+ second Snapchat stories of him singing.

You make a thread asking "How do you tell someone they suck at singing?" knowing that "someone" will probably see it.