There are a lot of these to be found on the web. I think this version is a bit beefier than some I have seen, so may be a little more robust.
This is a bit of a photo heavy post, so you may want to grab a cup of tea first...
To make this you will need a cereal packet, plain paper & patterned paper. Measurements are to fit a standard Post-It Note & the pencil I have used. You may have to adjust things slightly, depending on your choice of writing implement.
First, cut 3 10 cm squares from the cereal packet.
I have found that scoring lightly with a knife allows the glue to penetrate the cardboard a bit better, & make for a stronger finished item. If you are using dry adhesive, omit the scoring.
Glue the 3 layers together & leave under a heavy book for a few minutes.
When the the square is dry, cover it with the plain paper. Make sure it is a little wider than the square & just over twice as long. Sit the square to one side, allowing an even border on 3 sides. Draw around the square & cut as shown.
Stick the square in place with dry adhesive &, using an embossing stylus (or old biro) score around the edge, to make folding the paper easier & give a neat edge. Fold & stick the 3 small flaps first & then the large one. The side of the square where the flaps meet will be the BACK of the note holder.
To make the front piece, cut the patterned paper to the same width as your covered square. Depending on the thickness of the plain paper you used to cover the square, it may have grown quite a bit, so re-measure.
The pencil I used needed a 1cm square hole to middle piece back through the hole.
Apply strong adhesive as shown by the red lines. This is actually red line tape, which is super sticky, so that the front piece won't come away.
Orientate the covered square so that the flaps meet on the back, on the pencil side of the holder. This way, the major join will be hidden when the front piece is in place.
Carefully place the front piece in position & stick it down, folding the small flap around the back.
Finally, stick the note-pad in place. Don't just rely on the sticky back of the notes, use some double-sided tape.
Chevron patterned paper is from Pink Petticoat, colour is tangerine. Pencils were a lucky find in Sainsbury's last year - from the kids' party bag section.
Well congratulations if you've made it to the end. I think this post took longer to write than it did to make. Enjoy!