Stitch up through 1 and then down through 2.
Stitch up through 3 and then down through 4.
Up through 5 and down through 6.
Once you have finished your row you reverse your direction and “fill in the gaps”.
Stitch up through 7 and down through 8.
Stitch up through 9 and continue on down your column/row.
The benefits of using Holbein stitch is if you have to frog (remove) stitches, it is much quicker and it’s easier to work out if you have mis-stitched a border placement because it’s a quicker stitch.
The down side is Holbein tends to be a more “wobbly” appearing stitch than back stitch because the return row is pushing against the floss that has already been stitched. You can stitch either side on the existing floss so it appears closer to a whipped stitch and looks straighter. This stitch is not advised if you are using variegated threads as it is very hard to match the colour/tone transitions.