To make a decrease in basic Afghan stitch, you decrease in the first half of a row of basic Afghan stitch, subtracting loops from your hook; then, you work off all the loops in the second half as usual.
To practice making decreases, you can use a 16-stitch swatch. To decrease 1 stitch, work the first half of the row:
Insert your hook behind the first and second vertical bars in the row below.
You need to have 1 loop on your hook from the previous row.
Yarn over (yo) and draw the yarn through both stitches.
One decrease at the beginning of the row is complete.
Work in basic Afghan stitch across the rest of the row.
You should have 15 loops on your hook. (Remember, you decreased 1 stitch.)
Yarn over (yo) the hook and draw your yarn through 1 loop on the hook.
Make sure you draw it through only one loop.
Yarn over the hook and draw your yarn through the next 2 loops on the hook.
Repeat the preceding step and this step across the row until 1 loop remains on the hook.