Steam blocking

You may wish to block your motifs before doing the flat braid join. It's not required, but some people like to do so. Man-made yarns don't generally need much blocking, if any, but wool yarns or other natural fibers may need a bit more. My personal preference is to complete the afghan, then give it a nice steaming on a bed, smoothing all the seams and joinings out nicely and relaxing the stitch patterns a bit.
My favorite method is steam blocking. For straight-sided pieces, I use a piece of large gingham-check fabric to line everything up, and using rustproof pins, pin the crochet to a padded surface such as the ironing board cover or a towel on a counter. For large items, I use the bed.
Set the iron to steam, and if you have an iron that features bursts of steam, so much the better, but either way, use the iron to allow the steam to penetrate the crochet fibers without actually touching the iron to the crochet. If you prefer, cover the crochet with a pressing cloth just to be sure.
I then smooth the crochet with my hands until it all looks symmetrical and straight, adding more steam if needed, then let the crochet dry completely before removing.