I'm in the practicing stage and strongly suggest lots of practice if this is something you've been wanting to learn how to do.
Make a rough sketch of something simple on a doubled over piece of scrap cloth - thread sketching is much easier to do on a doubled piece of fabric or a quilt sandwich. I used my favorite coffee cup.
Put on your darning or free motion quilting foot and drop the feed dogs. This past post has more information on free motion quilting and how to get set up for it. Use a size 14 or 16 quilting needle.
Begin sewing! I have found it's easiest if you put your speed a little faster than you may be used to free motion quilting. Go over each line several times to give it that "sketched" look.
I'll show you what I've been working on when I get it done.
Happy Creating! Deborah