I know this is an old post, but I thought I'd comment in the event others want to know how to make editable decorations to apply on a cake. I've made 3-D numbers and letters of the size and thickness shown in the OP's photo. It's a pretty straightforward process. You just have to keep a few things in mind.
1. Fondant: fondant is NOT ideal for 3-D decorations as it will not dry hard. 3-D decorations made out of fondant will sag under its weight. Fondant is best used when the decorations are 2-D, thin and applied directly on the fondant or buttercream. And yes you can apply fondant decorations to buttercream. I use a tiny art brush to apply a little shortening or water on the underside of the cutout, then gently press to the side of the cake. Be very careful using water. You do not want the water to drip down the cake.
The buttercream should be chilled well before you attach. And since buttercream has moisture in it the decorations should be applied at the last minute.
Apply to fondant covered cakes the same way. If the decoration is large, you can use a piping gel to attach.
Fondant decorations can be made ahead of time and stored in airtight containers until needed.
2. Gumpaste: gumpaste is excellent for 3-D decorations. Gumpaste is made with confectioners' sugar, gum tragacanth, and glucose. Gum tragacanth is a plant resin that dries hard. It's sold under the brand name of tylose. You can make your own gumpaste or purchase it pre-made. It's edible, but I wouldn't call it delicious by any stretch of the imagination.
The benefit of gumpaste is its stability when rolled very thin. It can be rolled much thinner than fondant. Gumpaste is the preferred medium for flowers since it can be rolled thin and dries completely hard.
It also dries faster than fondant so do you need to work quickly. You should have all of your necessary tools and your workstation set up before you begin. Only remove the amount of gumpaste you need; keep the rest wrapped in plastic to prevent if from drying out.
Gumpaste can it be colored with gel food dyes or pedal dust. Whether creating delicate realistic flowers or big bold 3-D letters and numbers it is best to make gumpaste decorations at least 3 to 4 days before needed to allow it to dry through completely. Store in a box (I use a cake box) in a cool dry place. It's important to keep it out of direct light--especially sunlight. Light will cause colored gumpaste to fade.
3. 50-50 Fondant-Gumpaste blend: 50-50 blends are ideal for both 2-D and 3-D decorations. The inclusion of fondant allows for more work time. So 50-50 blend or ideal when making more intricate decorations such as figurines. 50-50 blend can also be colored with gel food coloring's and pedal dust. Like gum paste it will dry rigid. But it is a bit more durable than gumpaste.
Make 50-50 decorations ahead of time to allow for through drying. And store in a box, in dry cool dark place.
2-D 50-50 blend decorations can be applied to fondant or buttercream in the same manner as fondant decorations. A little bit of water or piping gel is all that's needed. Again if using water be very careful not to let any run down the side of the cake.
If applying to buttercream, apply at the last minute.
4. Securing 3-D Letter/Numbers to cake: when making thick 3-D letters or numbers there's two ways to insert supports.
A) Roll the gumpaste or 50-50 blend to the desired thickness. Cut out the letter with a cookie-cutter or template. Then gently insert a flat wood toothpicks into the center of the bottom of the letter. Make sure enough of the toothpick is inserted into the decoration to provide stability. I always use two flat wood toothpicks to ensue stability and balance. Dry the decoration on a flat surface for 24 to 48 hours depending on thickness. Dry in a dry cool area away from direct light.
B) Roll the gumpaste or 50-50 blend to have the desired thickness. Cut out two letters. Place one letter backside up. Gently press two flat wood toothpicks on to the letter. Leave enough toothpick exposed from the bottom to stick into the cake. With a fine brush, apply a thin coat of edible glue over letter. Then carefully place the second letter backside down over the first letter. Gently press. Gently smooth over the seam.
Dry the decoration on a flat surface for 24 to 48 hours depending on thickness. Dry in a dry cool area away from direct light.
Link to 2 recipes for edible glue.
https://www.craftsy.com/blog/2015/11/how-to-make-edible-glue/
Flat wood toothpicksI prefer are the ones used for eating tapas.
https://www.amazon.com/Don-Palillo-Spanish-Toothpicks-toothpicks/dp/B003L7N7I6
