The tools and materials you will need are:
Scissors
An old or broken CD
A small screw or button
Paint
A drill or a hammer and nail
Glue for reinforcement (optional)
Varnish or clear nail polish (optional)

Step 1: Shaping

Mark approximately where you want to cut out the shape of the dagger and how sharp you want it to be on the CD with a pencil or scissors.
Cut out the blade.

Step 2: Repeat and Paint

On the other side of the CD or on a new CD, cut out a makeshift handle.
Paint both the handle and the blade.

Step 3: Make a Hole

Drill or hammer out a hole at the bottom middle of the blade and the top middle of the handle. (Optional) Otherwise just glue the two pieces together.

Step 4: Join the Two Pieces With a Screw (optional)

Step 5: (Optional)

Paint over the entire dagger with varnish or clear nail polish to ensure that the paint does not get scratched.

Step 6: Finish

Add a proper handle or leave the dagger as it is. You can now add it to a wrist cuff or carry it in a sheath in places where real knives aren't allowed.