If you decrease at the beginning of the row:

Knit 1 stitch, Knit 2 stitches, lift the first stitch over the second, knit to the end of row.

To decrease at the end of the row:

Knit to last 3 stitches. Knit 2 together, knit one.

Follow these instructions if decreasing on a purl row.

Note the way the different decreasing techniques shape the garment and you can use this to be creative in future!