The first st of each round is done in the same stitch as the beginning chain 2 (all rounds start in the same stitch as beginning chain 2).

Make sure to join your rounds to the first hdc of the round (not the chain).

When changing color on the stripes, join the new color while completing the last hdc (half double crochet) of the previous round (by pulling through the new color when you have three loops on the hook in the previous color to complete the final hdc).