You don't have to get the spacing exactly correct. If your space is a little too big or too small, it won't really matter. You'll get better at judging the size of your beads as you work.
After you pull your thread up through the Stiff Stuff to the working side, pass your needle through the second bead you added. Pull snugly to secure the bead. Remember not to pull too tightly to prevent your bead embroidery medium (Stiff Stuff) from puckering as you work.

