Work all of the stitches in round 3 through the back loops of round 2, remembering to crochet over both the white and red yarn tails.
In crochet speak, the abbreviation that means "crochet through the back loops only" is "blo" so the pattern looks like this:
Round 3: (switch to red yarn) 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: