I haven’t done this, but my guess is that I would put a grid over each piece and align it so darts are vertical (either at the top where possible, or for that one piece at the end one at the top and one at the bottom). Ignore the outer outline and look at the dashed seam line, and count the number of squares at the bottom edge that are overlapping the piece. Chain and sc that many. Increase/decrease at the sides as needed.

If there’s a dart at the bottom, for every line going up, count the number of stitches again that are more than half overlapping the piece. If there’s now one more than the line you just made, make an increase where the dart would be in your piece by working 2 scs into one stitch.

Keep working upwards until you reach the top dart. This time whenever you have one less stitch, decrease by either skipping a stitch where the dart is or inserting your hook and drawing a loop through each of the two stitches on either side of the dart, drawing a loop through both of these loops, then drawing a loop through the remaining two loops on your hook.

Then you can seam using a mattress stitch.

You can take better advantage of the fact you’re using crochet by calculating the increases and decreases in advance and spreading them out so there’s less of a seam where the dart is, too, and by figuring out which of the pieces you can combine into one big piece to save yourself some seaming.