As some others have mentioned, fold the fabric in half only once and hold the template down HARD when you are cutting. As you are folding the fabric be sure to line up the selvage perfectly or you will get strange V-shaped strips... or if you fold it twice and it isn't perfect you'll end up with wavy strips.
Cutting is my least-favorite activity, so I was thrilled when I first found my strip template but it took a lot of getting used to in order to produce nice strips.
Also be sure that your rotary blade is super sharp--if you don't cut through both layers completely it's a hassle to separate them later.
I find that on slippery fabrics I have to re-align the template with each cut to be sure that I am not creating uneven cuts... it takes almost as much time as using a regular acrylic quilting ruler, but at least if I hold the template down hard enough the slit prevents me from cutting a curved line off to the right like I often do with the regular rulers.