Using two needles, one on each end of the thread, string three beads on one of the needles and slide them to the center of the thread. String a fourth bead, passing one needle through from left to right and passing the other needle through from right to left. String one bead with each needle, then pick up one more bead and pass one needle through from left to right and pass the other needle through from right to left. Continue for desired length of row. To work the next row, repeat as for the first row, stringing new beads only onto the right thread and passing back through beads from the first row with the left thread.
To make a row-end decrease, simply stop your row short and begin a new row.
See Right-Angle Weave