It is common for designs to require beads. Beading should be done after the cross stitching and backstitching.
The thread may be beading thread, floss that matches the color of the bead, floss that matches the color of the background fabric, quilting thread, or any kind of transparent thread. Each will produce a different effect, with a light-colored thread brightening the bead's color and a dark colored thread deadening the color.
The simplest method to attach a bead is with a half stitch or quarter stitch.
One method to keep the beads from drooping or sliding requires two strands of floss. Attach the bead using a half stitch, coming up through the first hole, through the bead, and down through the second (diagonal) hole. Then, come back up through the first hole, split the two strands of floss around the bead so one goes on each side, and go back down through the second hole.
Another technique, which is said to work well for a row, starts with the beads attached along the row with half stitches. At the end of the row, the thread is run back to the beginning by going through the beads, above the fabric.
Yet another method uses a full cross stitch. Attach the bead using a half stitch, then complete the cross stitch while going through the bead again. The order and direction of the two half stitches determines whether the hole in the bead points side-to-side or top-to-bottom.