Depending upon my project, here’s a little crochet tip that I like to do which helps to prepare my piece for seaming or joining.
I like to finish off my crochet piece by bordering it with one row of single crochet stitches around the entire square or garment. I even like to do this when not joining because it gives a nice, clean, professional look to your piece.
Basically, when you get to the end of your pattern, don’t fasten off, just continue around making a single crochet into every stitch within your project. That border row of single crochet will be where you will place your joining stitches to form your seam. Here are a few examples showing how to single crochet an edge. Complete your single crochet on working edge, when you get to the corner, complete three single crochet stitches in corner to keep edge from curling. Continue with single crochet on next edge by inserting your needle along the edge stitches. Keep them spaced fairly even so that your piece doesn’t curl and keep track of how many single crochet stitches you do so that you can do the same amount on the other side edge for a square, rectangle match the east/west sides with same number and the north/south sides with the same amount of stitches. Next corner do your three single crochet and do one single crochet in each chain stitch so you have the same amount of single crochet stitches across the bottom edge as you do the top edge. Continue with this pattern until you get back to the beginning corner of your top edge and fasten off.
If making a garment, you might only want to single crochet along the places that need to be seamed; along sleeve and side seams. Take into consideration the design and what works best for your particular pattern or design.