After completing your crochet piece, you have several options available as to how you want to join the seams of your project. Probably the most common method of joining is the WHIP STITCH. This joining method is not too bulky and creates a nice seam.
To begin, place your completed pieces right sides down and side by side. Now you need to thread your tapestry/yarn needle. I found this site that gives you a couple of different suggestions on how crafters thread a needle with yarn. Myself, I fold my yarn in half and pinch that folded edge tight while holding the yarn very close to the edges of my index finger and thumb. Now I just slide or work the eye of the needle onto the yarn and pull through the edge of the loop. You never knot the end of your yarn as you will just weave your tail through you stitches once you have completed your joining process.
Once your needle is threaded, leave about a 4″ tail for weaving and starting at one end, push the needle through both loops of your crochet stitches on both pieces that you are joining. Some patterns might tell you to join by only going through one loop on each side of your seam. Of course this is fine but take into consideration your finished project and if it needs to withstand heavy usage, I would recommend that you always insert through two loops on both pieces of your seam. Pull the yarn snug but not too tight (you don’t want puckering) so that the two pieces fit closely against each other. Draw your needle up and over and insert it into the next stitch. You will always be inserting your needle on the same side. For example, if you begin joining by inserting your needle from the right side to the left (or vice versa) , that is how you would continue whip stitching until you reached the end of your seam.
*TIP: Keep some practice swatches among your yarn stash so you can try out different joining stitches. For a more dramatic look, you could use a contrasting color of yarn for your piece, just make sure your joining yarn is the same type and ply as your finished crochet project.