I fold my fabric in 1/2 twice on the selvage---fold selvage to selvage then the fold to selvage (just like when cutting with regular ruler) still making sure that the fabric isn't twisted.
place the bottom line along the final fold of fabric with the uneven cut edge of fabric just past the "0" line. cut up the "0" line to get your first straight edge. If you want 2 1/2 inch strips cut up the 2 1/2 strip slot. If you are looking for squares or rectangles you then turn your mat 1/4 turn, place the line on the fabrics cut edge and cut our fabric in the correct slot. I don't cut triangles with this ruler.
I find that I have to hold the ruler firm to the fabric one slot over from the one I am cutting. My rotary cutter moves the ruler just a smidgen to open the slot for the blade. Just like any ruler if you start with this one finish with this one. If I am making a complete quilt it really doesn't matter if all of the blocks are off a 'smidgen', but if it is for a swap then I use something else.
Good luck. I know it took me awhile to get the hang of this ruler, but when I remember to use it my cutting is so much faster. I would like the bigger one, but make do with the one I have because of price.