The simplest way would of course be to just do the ol' cut-out crotch, snip off the foot area...and presto, armsleeves. They should fit nicely around the top of your arm, because it's the same area where your thighs used to go....

But I think I'm missing something here! Are you looking for a different sort of method...?

^Oh, I know you aren't! Sorry!

How about... taking some lace or ribbon, and weaving it through the top section...and tightening it a la corset style.

Just take a look at this for a little help.

Take some inch wide elastic and some inch wide hemming tape and place one on top of the other.
Put some tinfoil inside the glove at the top and place the elastic and hemming where you want it.

Place a towel over the top and iron.