Continue completing your boxes(as many as you need). To complete the box just repeat the steps, going from corner to corner and from top to bottom thru the X and from side to side thru the X. Remember that the lines you create on your drawing surface need to be repeated on your photo so when you go to transfer the drawing, your lines will be the same grid on both. You can continue gridding down as small as you need to transfer the details. When doing portraits, this is handy when you need to get eye, mouth and nose detail location.
If you need any help with this application of gridding just email me or call me and I will walk you through the steps.