Here is a quick and easy no-sew technique for joining the seams in your crochet garment by simply using the same working yarn and crochet hook as for the project.
Instead of sewing the sides or shoulder seams, you can use slip stitches (sl sts) to join the back and front pieces. Begin by holding these 2 pieces with the RS together and the WS facing you. Align the edges, so that the stitch pattern is matching on both sides. Now, join the seam with sl sts along the marked edge by working through the stitches of both pieces at the same time. Depending on the pattern, you can place chains over chain stitches along the seam to compensate for the stretch (as I did here in granny stitch pattern).
You can also use single crochet stitches (sc) if you need to create a base row/round after joining your pieces. For example, I connected the hood to the body with sc, so I could add ribbing to both pieces in the end and eliminate sewing of the hood. In the situation with the hood (or when you are setting in the sleeves), your stitch pattern on the back and front may not be aligned due to the different work direction...so, you will need to use pins rather than stitch references for aligning your pieces.
Simply position and pin the hood around the neckline of the cardigan, spacing their edges evenly between the markers and making sure that the main points are aligned and centered. The rest is easy, just work sc through both pieces at the same time (as indicated in the pattern), removing pins as you go...and voila, no need to sew!
