To make this easier, cut squares from cardboard, cardstock, template plastic or whatever you have on hand in the following sizes: 2 1/2", 2", 1 1/2". These are the most common sizes.
Place the 1/4" line of the ruler corner to corner and cut the paper/plastic along the edge of the ruler. Label the size of the square. Put these in a baggie or container so you can find them easily.
For a square where you would normally stitch corner to corner, simply place the "take away" template on the corner of the fabric and line up the ruler. No more second guessing if you have the ruler lined up correctly.
Now take the template away and cut the fabric. After you get accustomed to doing this and where to line up the lines of the ruler, you won't need to use the take away templates.
I want everyone to be successful with the Farmer's Wife, not frustrated! Are you confused about what size take-away template to use? A 2" finished half square (would be cut with a 2 1/2" strip), so you would use the 2 1/2" template. If the strip you are cutting away the fabric on is 2", you use the 2" cut away template (this would be labeled as a 1 1/2" finished half square)
If you have any questions, just ask. Someone else probably has the same question.