Blackberries have a two-step growth cycle. During the first year, the primocanes grow. Those survive the next winter, and in the next spring they become blooming floricanes, which bear the berries. The exceptions are a few modern types that compress the cycle into one season, such as primocane bearing Prime Ark Freedom.
Some varieties are too vigorous. My favorite is Triple Crown, which must grow 10 or 12 feet tall if I let it. Then they fall over. If they are topped when much shorter, they will grow lateral canes and remain much shorter. The berry production is supposed to be much better too, on these laterals.
The pruning is very simple. Books call it "pinching", but if the tip is bent, it snaps off. No knife or pruning shears are needed. There are various recommendations about the height - 2 to 4 feet. I'm aiming for waist high, about 3 feet, but did these a little shorter to catch them while I was there and remembering.
Snapped!