Q&A for How to Thread a Needle and Tie a Knot

Start inside of your hem. Hide your knot in the fold of the hem. Right next to where you started, on the other side, pick up with your needle just a few threads in the fabric. Then go right back down into the fold of the hem. The needle will follow that fold, so the thread stays hidden as you make your seam. Pull your needle out of the top of the fold. Right next to where your needle comes out of the hem, threading needle back through your stitching. Repeat until you finish your seam, then tie a knot close seam, then pull the excess thread back through your stitches without piercing the front of the fabric. After about an inch, cut your thread off your needle.

Tie a finish knot for the blanket stitch, thread needle through the open loop making sure to hold the needle on one side of the loop while grabbing the thread still on other side of loop. Pulling on the thread from needle will tighten the loop you took the needle through, leaving your needle on one side and a new loop. Thread the needle back through the new loop and pull tight. This will give you a sturdy finished knot but also leave it as a small inconspicuous finish.

If you mean end-to-end, to make the resulting length the sum of the individual threads' lengths (minus the amount used by the knot), use the square knot or the surgical knot - see tutorials online for instructions on tying. The resulting knot won't pass through fabric easily as does continuous thread, unless the fabric has a very open weave.

Fold the thread at the eye of the needle and apply a little beeswax to the thread. This will hold it in place while you sew.

No, because then you can't get it undone. However, you can tie several times to prevent this issue.

You can color just the tip of the invisible thread to secure and see them. Once threaded, you can clip the colored bit. Use lighting to your advantage so the light and shadow helps you see the transparent thread.