Fold your fabric just enough to accommodate your pattern. Pin with the longest part of the pattern right along the fold. Carefully cut around the pattern. Then do the same thing again so you have two pieces of cut fabric.
How you fold your fabric and where you cut will depend on the size pants you're making. With toddler sizes, you can use a mere 2 / 3 of a yard and fit both legs within a single width of fabric. With larger sizes, this isn't possible because the crotch makes the pattern too wide to fit twice. This is where a non-directional fabric saves you some money. You can flip your pattern and place it on the other side, sliding it down until the two crotches meet (boy, does that sound dirty) and waste less fabric.