Stitch up through 1 and then down through 2.
Stitch up through 3 and then down through 1.
Continue up through 4 down through 3, up through 5 down through 4 etc.
The benefits of backstitch is that it tends to be very straight. If you are using variegated floss this is the stitch I advise using.
The down side of this stitch is that more floss will used than in Holbein stitch.