I use a ruler usually
Insert the paper as far as you can without forcing, then take a narrow metal ruler and insert between the clear plastic and the lable. Push on the ruler so that as you move it down, the paper also moves down - after a bit of practice, it takes only moments to get a perfect lable done.
Another tip is to make sure you have the folder open "spread eagle" - generally the plasic is stretched slightly when the binder is closed.
The second method I use is to print the lable on plain paper (slightly smaller than the spine), use a decent glue to stick it down, then contact over the top