You will a square sheet of pattern paper to start with. A 4" x 4" square is shown here, which is a good size.
Fold into half.
Unfold and turn the paper 90 degree and fold into half again.
Unfold and fold the paper diagonally as shown.
As you unfold the paper, you will get a cross and a diagonal line as shown.
Reverse the paper and fold the corner inward towards the center as shown.
Repeat all 4 corners.
Then unfold one corner where the diagonal line doesn't fall onto.
Start to fold inwards using the diagonal line as a guide.
As you push in the adjacent sides, you will start to see how the 2 sides collapse inward.
Then push them inwards and down as shown.
As you press down everything, you will get a diamond shape as shown.
Flip over and this is what you will see.
Open up the book corner slightly.
Then fold and push in the flagging corner into the slot.
And this is how it should look like.
Give a good press and there you go, you have made a book corner.
Add some embellishment if you wish to. This is what I've done to another book corner.
This is how it will fit into the corner of a book, which you can use it to fit into any other pages within a book as a bookmark. Enjoy!