If you are sewing by hand, here is the Wiki How for hand sewn seams.

If you are using a sewing machine, make sure the bobbin is filled with your chosen thread and line up the fabric for a 1/4" seam.

Sew three (3) sides of your fabric, pulling out the pins as you go along.

When you complete your three sides, backstitch about 1/2", pull the fabric away from the sewing machine to give yourself a long tail of thread, and then snip the thread.