For the regular size muffin pans, you will want to cut at least 5 inch squares. For the jumbo size muffin pans, you will need to cut at least 6.5 inch squares.
You will want to cut all of your squares before you begin mixing up your muffins--you will fix one wrapper at a time once you start the process of filling the cups with batter.
First, place your square over the muffin cup until it is right in the middle, leaving the same amount of paper hanging over each side.
Use your fingers to press and create a fold where that happens. You will want to be sure to crease these areas really well.
You want to be sure that the folds are creased really good to prevent batter from seeping behind the fold. If this happens, your liner will not look as pretty once the muffins are baked. The trick to this is practice! Having an extra hand helps, but it will just take practice to crease and fill the cups neatly!