selm

feline fanatic has and excellent description of how to do it. I will add that if the thread runs out in the middle of the quilt, you can "splice" the thread this way: Start your new thread several stitches back on the line of stitching(no more than 3-4 stitches). bring bobbin thread to the top and lay to one side. Stitch over the old line and beyond several stitches(or finish your row) then go back and snip the tails of threads. Important - I always use small, curved scissors and Don't cut with the scissors. Instead slice the thread with the v where the scissor blades meet. This is like using a razor and keeps you from cutting a hole in the fabric which can happen if you use a cutting motion with the blades. Sometimes you may have a tiny stump of thread after trimming. This usually sinks into the quilt with time and washing.
Also, if you are doing a pantograph, be sure to take any tails and lay out of the way of the upcoming pattern. You can take out tails if caught in the pattern stitches but it can be difficult if they get caught in the new stitches. Sometimes I deliberately make sure the tails are quite long so I can lay then aside and wrap around a pin to hold in place until I pass that section.