Work all of the stitches in round 3 through the back loops of round 2.
In crochet speak, the abbreviation that means "crochet through the back loops only" is "blo" so the pattern looks like this:
Round 3: sc blo in the second st from the hook, 2 sc blo, dec blo, [3 sc blo, dec blo] x 5, ch 1 and turn (24)
What's a back loop, you ask? The back loop is the side of the horizontal V that's further away from you. So instead of putting your hook under the entire horizontal V, put it only under the side of the V further away from you like this:
The invisible decrease stitch, which is what we normally do for amigurumi, can be tricky to do in back loops only so we recommend doing a regular decrease stitch instead. Watch how to do the regular decrease here: