Once you have your single thread (or 2-3 strands), fold it in half so that the ends are together and you have a loop at the other end. Thread the two ends through your needle.
Now take your fabric and come up from the front into your first stitch, but don’t pull the thread through yet. You should see your loop end. That is it! Use this loop to fix the thread and cover by the next stitches 😉 I do it all the time when I need 2, 4 or 6 strands 👌
Here is the video tutorial: https://youtu.be/q81QAe_mqJk