I've been getting envelopes in the mail with large transparent windows. They make great bookmarks.
Step 1: Open the Envelope
Slit the top and right folds to open the envelope..
Step 2: Mark the Cut Lines
Caution: You want to cut only top piece of paper in this step. The bottom part (the back of the former envelope) will be untouched after this cutting.
"X"s in picture indicate the waste part of the cut - which you will throw away. I'm sorry they aren't as prominent as they should be. You will cut the plastic loose from the paper in the top and right side leaving the plastic attached along the left and bottom..
AGAIN: Cut only the top piece of paper.
Step 3: Cut
Should look like this. At this point you could use the bookmark but a couple more features will make it easier to use.
Step 4: Extra Features
Fold the upper left corner of the top up just a little. Fold the upper right corner of the plastic up just a little. You may have to crease the plastic a little with your thumb nail. These two slight folds will enable you to turn pages with one hand.
Next cut off the lower left corner of the envelope to make it easy to store several pages in the bookmark.
Step 5: Illustration of Bookmark in Use
Just in case it wasn't clear how to use the bookmark, see the picture. The cut-off corner holds the upper left corner of even-numbered pages (while I drift off to sleep).