One of the most basic stitches in embroidery and beading is the backstitch. The backstitch is named that way because you are stitching backwards, towards your starting point. This is not the same thing as a stem stitch, where you start the next stitch at the halfway point of the previous stitch. See the accompanying slideshow for a pictorial.
Thread a needle with the desired length of thread and tie an overhand knot in the end. Insert your needle from the back of the item to the front.
Slide three beads onto the needle, and bring the needle down through the front of the item to the back.
Insert the needle from the back of the item to the front just behind the third bead and string the needle through the third bead.
Slide three beads onto the needle and insert the needle and repeat the process. If you need the beads to sit tightly, use two beads at a time instead of three. If your bead holes are too small for a standard needle to go through twice, use a specialized beading needle or try beads with larger holes. This completes the backstitch.