Fold your fabric selvage to selvage and place it on the cutting mat with the fold at the bottom and the selvages at the top.

Note: You will not be using the grid on your cutting mat to measure where to cut so it doesn't matter where you place the fabric in relation to that grid.

 
Place the 12.5-inch square ruler along the bottom of the fold, with it's left edge about 1.5-inches from the left edge of the fabric. Make sure that the ruler is exactly straight against the fold as show in the picture below:

I lined up the third white line from the bottom on  the 12.5-inch square ruler perfectly with the fold. Make sure you line this up very carefully (it doesn't matter what line you use on the ruler, you're just getting a straight line, not taking a measurement.)

Place your 6.5-inch x 24.5-inch ruler vertically against the left hand side of the square ruler. Be careful not to jostle the square ruler as you place the long ruler.

Place your hand on the long ruler to anchor it and remove the square ruler.

Pick up your rotary cutter and cut along the right side of your long ruler. Your fabric is now "squared up" with the fold meaning it's been cut at a perfect 90 degree angle.