Attaching set-in sleeves (the easy, cheater way).
Okay, so it’s not really cheating. It’s a perfectly legitimate way of attaching a sleeve, and it is way easier.
However, a set-in sleeve is a set-in sleeve because you’re supposed to be setting it in, and with this method, you don’t.
Anyway, I’m splitting hairs.
Keep in mind, though, that this will only work if you haven’t already sewn up your side seams. If your side seams are sewn, you’ll either have to cut them open, or use the other method.
And if your garment doesn’t have side seams that line up with the sleeve seams, you’ll also have to use the other method.