How do I change colors when knitting only a single row of a different color?

I'm knitting a hat that requires only one round in a different color, It seemed like a simple pattern, but I'm running into a few problems with it. When I knit the single row in the other color, I never actually join the round which leaves a gap at the seam. How can I correct this. Also, I tried not cutting the yarn at the end of the round, then using it again when I needed it a few rounds later, but it seemed to pull funny. I'm trying to avoid a lot of loose ends to weave in and thought I could save myself some headaches by doing it that way. any suggestions?