Can you take it in to your local quilt shop and ask them to give you a quick lesson? My shop is always very helpful!
Be sure you are holding your ruler straight and in place (it can slip very easily!) , and you will probably have to reposition your ruler to cut the third side of your triangle.
Hang in there! You will get the hang of it! My mom was a long time template user and was very dismissive of and frustrated with rotary cutting at first, but once she got it, she never used a template again!