First of all, the square knot (Reef Knot) is primarily a binding knot. That is, it’s used to secure a rope or line around an object. In Pioneering, its most basic function is to commonly finish off a strop lashing, Mark II Square Lashing, Filipino Diagonal Lashing, West Country Round Lashing, and both the West Country and Sailmaker’s Whipping.
A square knot is formed by tying a left-handed half knot and then a right-handed half knot, or vice versa, and is regularly taught and learned by passing on the familiar, “right-over-left, left-over-right,” or “over-under, under-over.” However, frequently in the midst of tying the knot, it can be easy to forget what was done or how it began, e.g. right over left, or left over right, and which end was used. Therefore, here’s the sure-fire way to ALWAYS tie it right, every time! Simply tie a half knot, then tie another, BUT, keep the ends on the side they’re already on! In other words, don’t cross them over. Stay on the same side! One running end is nearer to you (in front) and the other is farther away (behind). Keep them there as you begin to tie the second half knot. Simple! In the illustration, red and blue ropes have been used and are referred to for clarity sake.
The beauty of this simplistic, little approach is you can see how to tie that second half knot without knowing (or remembering) how the first one was tied.