The body of the sweater consisted of three pieces: two trapezoidal front panels and one rectangular back panel.
I started crocheting the front panels at the shortest edge, N. I increased gradually every few rows, always on the same edge so that one side would slope and the other side would remain straight. I continued increasing until the width of the front panel was equal to W and the height was equal to H.
This is what I ended up with:
On each side of the sweater, I started at the shoulder seam and measured down the amount given by WS to determine how much space was needed for the sleeves. I marked that with a stitch marker and then sewed from there down to close up the side seam.
After finishing the ends, I turned the sweater right side out. Using a contrasting color of yarn, I added a row of single crochet to the ends of the sleeves. I did single crochet around the edge of the body of the sweater as well, and while doing that edging I made the four ties that keep the kimono shut. Whenever I reached one of the shoulder seams or the bottom corners of the front panels, I made a chain of about 20 stitches, slip-stitched back down the chain and then continued on the with the edging: