Work a group of vertical stitches evenly spaced, using a strong thread and ensuring they are taut.
Introduce the second thread by bringing your needle up just to the right of the rightmost thread. You will work blocks of cretan stitches diagonally from top right to bottom left.

Weave your needle horizontally under the vertical foundation thread and over the second foundation thread.  Bring your needle back under the second foundation thread, keeping your working thread above the weaving.  Cross over the first horizontal stitch and also the first vertical stitch.

Take your thread under the first vertical thread and over the previous horizontal stitch.  The motion is similar to a herringbone stitch: it should form a figure of eight.

Repeat the figure of eight movement.

Continue in this way until you have completed four cretan stitches.  You will need to use your needle or a mellor to push the stitches together and keep them even. 
Start a new block by taking your thread under the second vertical thread, and over the third one.  Reversing the direction, bring your needle under the third thread and diagonally over the horizontal stitch to form a figure of eight, as before.

Continue working the cretan stitches in diagonal blocks of four, using your needle or mellor to keep the positioning even.  Finish your thread at the leftmost vertical thread.

Start a new diagonal row on the right side, working it as before.
Continue working diagonal rows until the area is complete.