Step 1: Pick out a pair of shorts that fit you well and put them on top the jeans you want to cut. Fasten them together at the crotch with straight pins (or anything that will keep them from moving honestly).
Step 2: Put the ruler on top of the original pair of jeans (angled diagonally) and make a dotted line along the jeans with Tailor's Chalk or any other writing utensil that you can see and is washable (I used a pen).
Step 3: Take the ruler and measure down one half inch from the original dotted lines you made. This ill be the actual line you cut from, if you want your shorts shorter when you're finished, simply cut them again, but remember they will get shorter once washed and frayed.
Step 4: Using the mark you made as a guideline cut your first pant leg.
Step 5: This step is different from the website, but I feel it works better. Fold the cut leg over and use it as a guide to cut the other pant leg (that way even if you messed up your first cut a bit, both legs will be even).
Step 6: Ta-da! Throw these babies in the washer and dryer for the frayed ends to appear right away, or cuff them and wear them and wash them when you're done (that's what I did).