Leave at least a 1" tail and when you are finished your backstitching, thread that tail into your needle and work on the back of the stitching.

Hi typin - If I understand your question you are wanting to start a single thread with nothing to anchor it to. Is this for words or just backstitching like vines and stems? When I am using a single thread for backstitching letters or stems and vines I bring the thread up in the correct spot and hold about an inch tail on the underneath side with my left pointer finger while I go down at the top. Then while holding the single thread on the bottom I repeat the stitch - just go over where you put the stitch. Pull snuggly but not tight enough to distort the thread. If you want to at this time you can take your needle and just run through the threads on the underneath side before you go on with your next stitch. From this point you should be able to use your single thread without any problems. When finished you can trim the inch thread to where you are comfortable with it. I hope this makes sense to you.
cheryl

I would tie a knot.

I would leave a long enough tail of thread hanging underneath to be threaded in a needle and woven under the stitches created by the backstitching on the underside. I agree with the second stitcher clpatt123