Draw a series of horizontal lines, using your straightedge as a guide. Space them equally far apart. For example, leave 5 millimeters between each line.
Turn the straightedge vertically, and move it to the left part of the grid. Draw vertical lines connecting every second horizontal line.
Move the straightedge to the right, approximately four times as far as the distance between each horizontal line. With this example, move the straightedge 2 centimeters.
Connect the same horizontal lines that you connected in Step 1.
Repeat the previous two steps until you've added short vertical lines across the entire horizontal length of the wall.
Return the straightedge to the left side of the grid. Position it between the first two sets of vertical lines.
Draw new vertical lines connecting each horizontal line that you've not yet joined.
Repeat steps 3 and 4 with the new vertical lines until vertical lines join all of the grids' rows.