Crochet increases and decreases
An increase works two stitches into one base; a decrease joins two base stitches into one top.
Work the technique
US sc increase: make two full sc in the same stitch. sc2tog: draw up a loop in each of the next two stitches, yarn over and draw through all three loops.
Try it on a sample
On 12 sc, [sc, increase] 6 times makes 18. On the following round [sc, sc2tog] 6 times returns to 12.
Check and recover
Each increase adds one and each sc2tog removes one. Count the two consumed base stitches together so an extra skip does not double the decrease.