To get the triangle shape, you have three options:
1) Print from template
2) Draw it
Next, paste the triangle decorative paper onto triangle card.
Then decorate it according your perspective; ribbon, sticker etc. Instead being bookmark, it also can be a decorative for the book and we also feel more motivate to read.
Tadaa, it’s done.
You also can create a paper pocket at the last page of book from the library to save this bookmark from lost.