Before I discovered a wonderful gadget called the Walking Foot, I used to tie all my quilts. It works well if you are using a thicker batting, or making it more like a comforter than a quilt. It doesn't hold up as well in the wash, especially on baby quilts that are washed frequently.
I also used 6-strand embroidery floss (easier to thread the needle than yarn). As far as a method, not much different than sewing on a button, just tie it on top and leave the ends however long you want. I usually left less than 1 inch. How far apart depends on your batting and the design.
Personally, for a king size bed quilt, I'd quilt it, rather than tie it. Just my preference.