Binding off in knitting can be one of the most exciting steps of the project, since every stitch takes you closer to finishing. While the actual motion of binding off (alternatively called casting off) is very similar to knitting, there is an important difference.
A standard bind-off row naturally has less elasticity than your typical knitted row, so it’s important to keep the tension nice and loose. Otherwise, you’ll end up with a very tight edge, possibly even causing the fabric to ruffle or pucker.
If you notice that your bind-off is too tight, try using a larger needle just for the bind-off row. Usually going up one or two sizes will be enough to make a difference. Avoid making your bind-off TOO loose however, since that could also cause ruffling. Experiment with a small swatch until you find the right solution, then you can return to your project and work a beautiful bind-off!