How to back stitch...

Every square of the grid represents two threads of the fabric in each direction

the needle comes out of the fabric at odd numbers and down into the fabric at even numbers

working from right to left, the needle travels one block (two fabric threads) to the right on top of the fabric

the needle comes out of the fabric at odd numbers and down into the fabric at even numbers

and two blocks (four threads) to the left at the back.