As for places where you are instructed to make a knot at the end of stitching (such as the step I just finished, sewing buckle loops):

Don't try to make a knot in the thread itself, as you did at the start of your stitching (before thread entered fabric). Instead, turn to the wrong side of the fabric and take a very short stitch through one layer, so it doesn't show on the right side. As you pull this stitch through, insert your needle through the loop of thread so that when it's pulled tight, you have a simple overhand knot. Repeat this so you have two overhand knot/stitches close together. Then, I insert my needle below one layer of fabric and pull it out a few inches away before cutting off the thread. This captures the last loose inch of thread between fabric layers and decreases the chance of those overhand knots ever coming loose.

Hope this helps!