The square knot and the half knot are the same movement; the difference is whether you finish it. A column of square knots is flat, a column of half knots twists. Both are tied on four strands: two working strands on the outside, two fillers in the middle.
The half knot
Take the left working strand across the front of the two fillers and under the right working strand, so it makes a shape like a 4. Take the right working strand behind the fillers and up through the loop on the left. Pull both working strands outward, then slide the knot up snug. That is a half knot; the left strand is now on the right.
The square knot
Tie a half knot from the left, then tie the mirror image from the right: the right working strand across the front of the fillers and under the left, the left strand behind the fillers and up through the loop on the right. Pull snug. The knot has a small bump on the side where the first half knot started. Every square knot in a sinnet should start from the same side, or the bumps alternate and the column looks uneven.
Keeping a sinnet flat
A square knot sinnet stays flat when each knot is started from the same side and pushed up firmly against the previous knot before the next one is tied. Hold the fillers taut with one hand while the other pulls the working strands; the fillers should run straight through the middle and the knots should be the same height all the way down. A square knot in 4 mm cotton is about 1 cm tall, so ten knots per 10 cm is a good check.
Making a spiral
Tie half knots only, all from the left (or all from the right), and after every four or five knots the column turns a quarter turn on its own. Let the work rotate in your hands rather than twisting the cords back. A half knot is half as tall as a square knot, so a 15 cm spiral in 4 mm cord is about 30 half knots. The spiral direction reverses if you switch the starting side, which is used on purpose in some patterns.
Tension
The multipliers used for cut lengths assume knots pushed snug against each other. If you knot loosely, the arms come out longer and use less cord; if you pull very tight, the reverse. Tie a test sinnet of ten knots in your cord and measure it before starting a piece with a set hanging length.