would like to know how to sew a hankerchief?

Answer 1

A handkerchief is about as easy as it gets. Cut your fabric in a square & hem the edges with a narrow hem. A pocket-sized handkerchief is about 12" square or even smaller, a bandana-sized one more like 18", but you can make it any size you like.

Answer 2

There's a roll foot on the sewing machine that you can use with the zig-zag stich to get a really narrow hem, but it's kind of weird on the edges.

If you are going to do it by hand, try using an iron to crease the edges over -- it will help the hem stay in place. Also, curve the corners instead of doing 90 degree corners.