--- --- --- --- --- ---
|                     |
|                     |
|                     |
|                     |
|                     |
|                     |
|                     |
 --- --- --- --- --- ---

Start with a small square piece of paper (you'll decide how small).

 --- --- --- --- --- ---
| '                    |
|   '                  |
|      '               |
|         '            |
|            '         |
|               '      |
|                  '   |
 --- --- --- --- --- ---- 

Then fold top right corner down to bottom left corner so it makes a triangle.(see below)

|* 
|   '
|      '
|         '
|            '
|               '
|                  '
** --- --- --- --- --- ---*

next pull the two points (see *&*)one at a time to the same bottom corner (see **) (so all your corners will collect at the same corner as at first step) now you have a small sqare.

 --- --- --- ---
|              |
|              |
|              |
|              |
|              |
 --- --- --- ---

unfold those flaps and put the now free corner so it folds into a triangle touching the longest line of the bigger triangle.(see below)

*
| '
|   '
|      '
|---------  '
|* '      |   '
|    '    |      '
|   ** '  |         '
 --- --- --- --- --- -**


Now, tuck the previously untucked corners( see * & **) under the new half triangle et voilà.
It should be a small square with a diagonal triangle flap where you tucked the two corners.
This allows for it to hold to a corner of the page you desire to bookmark. You can make lots of these and even colour code them if you like. This bookmark beats those stick-it things that protrude from study books - I don't like those, pretty as they are - plus these are much kinder to books (especially old books).