There are many different ways to seam knitting, but there's one surefire, quick and easy way to handle small seams.
There are probably as many different techniques for sewing seams in knitting as there are for weaving in ends. The key to sewing seams in knitting is to get the two pieces lined up properly and sewn together securely so that your finished piece looks good and holds together through use and washing.
The easiest way to do that is simply to turn the knitted piece inside out and line up the two ends you need to seam together. Or sew into the first row of stitches on each side from the right side of the project, ignoring the cast-on and bound-off edges. If your project needs to be blocked before you knit up in order to make the sides you're trying to knit together the same length, please do so.
When everything's in order, thread a yarn needle with the thread you are using and weave back and forth through the loops of the cast on (or bound off, or knit) edges of the pieces. Make sure you're going through both pieces of knitting on each pass. Keep working in this manner until you reach the end of the work.
If you want a very secure seam, you can sew back through the stitches in the opposite direction as well. This is an easy seaming method that blends in pretty well and doesn't add a lot of bulk to your seam. It's the perfect quick seam for little projects like Valentine's wristwarmers.