Knotted diamond stitch is a pretty border stitch. This stitch might look difficult but it is not.
This stitch is stitched between 2 parallel lines. Bring the needle up at the end of line #1 and make a straight stitch by going down at the end of line #2. Pick up a few strands and come back up.
Make an inverted U with the working thread as shown below.
Lace the needle through the straight stitch. Bring the needle under and then over the working thread as shown below.
A knot is formed at the end of the straight stitch.
Make another inverted U with the working thread and repeat the previous step. This time the knot has to be placed at the other end. So using the needle, push the knot while pulling the thread.
Another knot has been made on the other side. A second stitch has been added parallel to the first one.
Go down near the knot and come back up a little further away.
Make a loop that resembles letter U and lace the needle through the second stitch and also through the loop as shown below.
A knot has been made in the middle now.
Repeat the steps.