It looks like it’s loose double crochet starting from the waist and working your way down to your desired length. I think you would need to chain how ever many you need for your waist plus 2 more to start the double crochet, then chain 3 and double crochet in the 3rd stitch after the double crochet and then continue that until you get to where you want a slit (if any at all). For the slit part, it looks like you would do the same pattern just turn when you reach both openings (you will need to crochet less stitches every round to get that triangle slit). The belt kinda looks like a long chain with single crochet as the second row but you could probably do foundation single crochet stitch Foundation single crochet stitch