Your local hobby shop will sell cardboard numbers and letters in a range of sizes. After cutting the back side off, you can use the cardboard structure as a mold to cast concrete numbers or letters.

Materials and Supplies:

Rubber Gloves, Safety Glasses, Dust Mask
Cardboard Letters
Clear Lacquer / Shellac / Paint
Wire Mesh for Reinforcement
Mixing Containers
ShapeCrete

Tools:

Utility Knife
Wire Cutters
Diamond Hand Sanding Pad

Step 1: Cut the Back Side Off

Use a utility knife to cut the back of the letter off.
Pull out the cardboard structure inside the number.
Tape together any splits in the cardboard.

Step 2: Seal the Form

Spray the inside of the form with clear acrylic paint, shellac, or seal it with something similar.

If the cardboard is unsealed, it will soak up water from the mix while it cures, which usually leaves a chalky finish on the piece and can cause small surface cracking.

Spray a few coats of paint in the form and make sure it's fully dry before casting.

Step 3: Bend Wire Mesh Reinforcement

To reinforce the piece, we're using wire mesh screen, rolling it into a tube, and then bending it to fit in the form.

Cut a long strip of screen about 1" wide.

Bend it into a cylinder.

Bend the cylinder to match the profile of the letter, leaving a gap of at least 1/2" on all sides.

Step 4: Mix and Fill

Mix ShapeCrete to a Castable Consistency, and Add Pigment if you want it to be a color. Start filling and shake the form around to level out the mix. When it's about halfway full, stop, and place the reinforcement inside.

Float the Reinforcement: Press the reinforcement down into the mix, keeping it about halfway through. Then top off the form.

Screed the Surface: Use a flat stick or a ruler to scrape the top and remove any excess mix.

Wall Supports: Because the walls of the form aren't very rigid, they'll bow out a little bit when the mix is added. You can use something like 2x4s to help straighten the walls out again.

Cure: Cover the piece with plastic and let it cure for 24 hours.