While I have used extremely tedious manual method in the past, I recently worked out how to use a modifier to quickly produce the same effect.
Once you learn this process, you can apply it to any other font or decal and make your own designs.
Setup The Text
Since this method is relatively quick you can go about making text shadows two ways.
Make all a shadow for all letters, numbers and symbols you may need all at once
Make a shadow for only the text you need for the decal / project you are working on.
The first locks you into a ratio of letter to shadow, but lets you get a lot of the prep work done ahead of time. The second option will make you duplicate a lot of prep work.
When inserting text using a font, you can make all your letters on one row, or break them into rows with a few letters each
It is helpful to expand the distance between letters but increasing the Spacing option by a little bit.
Convert Text To Mesh
If you are working on a decal you can skip this step, however when using fonts, this is where most of the prep work is done in order to ensure a clean final mesh.
To convert a text object to a mesh object, ensure you are in Object Mode and then press CTRL + c.
When you first see the mesh generated by the converter, it will be highly triangulated and have some oddly placed faces given the vertices present. Press ALT + j to convert some of the triangular faces to quadrilaterals.
As you can see here, a lot of the odd triangular faces were cleaned up, however now we have some oddities, particularly with the X and Ts.
With some careful re-defining of the faces, we can now ensure that all the edges and vertices are used by faces.
Making the Shadow
Now that the mesh is prepped, we can make the shadow.
Duplicate the mesh object and move it slightly below the original mesh. Paint the top object one color and the lower object a different color. This provides us contrast to help make things easier later.
Select the bottom object and move it in the x and y directions by the same amount. From the top-down view you should see something like the images below. Note that the object is translated in the X and Y directions by the same magnitude (0.03m). This ensures that things are setup for the shadow that I am creating, however you can play with these values to make the type of shadow you want.
Extrude the top object down below the bottom object.
Select the shadow (former bottom) object and set up a boolean modifier as shown below. Note that in the Object (Ob) box, enter the name of the top object. You can find this in the Transform Properties window when selecting the object. When you apply the modifier and you should see something like the image below
Delete the vertices shown in the image above. Note that these vertices align with the extruded object. The only vertices that you want to keep are the ones that were in the original vertical position.
If you zoom in on the newly modified shadow mesh, you will see that there are some oddly placed vertices. Clean these up as shown in the images below.
Delete the bottom of the extruded object so that only the original mesh remains. Merge this object with the shadow and scale the new mesh in the z-axis by 0. This will force the mesh to interlock. Remove duplicate vertices and you now have a completed shadow!