Bring the needle through at A and insert the needle at B.
Bring the needle out at C (the distance between C - A and A - B should be equal).
Re-insert the needle at A, making sure the needle is inserted exactly in the hole previously made.
Bring the needle out at D, keeping the distance between D and C, the same as the length of the previous stitches.
Re-insert the needle at C in exactly the same hole as previously made.