This page was my favorite tutorial for intarsia in the round. It covers two distinct techniques for “joining” your work in the round. Intarsia in the round is not truly in the round, as you are still turning your work to work alternating right side and wrong side rows. However, at the beginning and end of each row, you join the ends of your rows so that they’re linked in the round.
In this hat, only the body used intarsia. The ribbing was knit completely in the round. I started the intarsia with the round transition/join between the off-white and salmon pink yarn, initially. As I improvised the geometric design, I introduced the navy yarn on one side of the join, so the join was now between off-white and navy. I then added the bright blue yarn exactly at the join, and since I wanted the shape to grow as a diamond, I started shifting the join over by one stitch every row. The join was then between the off-white and bright blue regions.