Who are "they"? :P
Everyone can help out by adding to the wiki. I haven't made anything in a while; I didn't realize there wasn't a more detailed guide until I looked. I may start one if thats the case.
But the process is pretty straightforward:
1.) design your unit -- think about abilities, how it fits in with everything, shape, size etc.
2.) create a model of your design -- anything can be used for this, if you're new you might want to start with Blender as its free, and has a lot of bells and whistles that the really expensive software has. However, just remember that this comes with a somewhat steep learning curve, and may take some time getting used to. ***Keep in mind how you're going to animate the pieces for later***
3.) create a texture mesh of the model -- map out the 3d shapes to a 2d square surface. Whatever tool creates the model will have something for this. This is usually not too terrible once you get the hang of it, but its helpful to think about how the surface of your model will be applied to a 2d plane while creating it to make this step easy.
4.) make the texture -- using whatever image processor you want. Personally I use Gimp -- similar to Blender it has a ton of bells and whistles like Photoshop, without any of the cost. Theres a few different texture layers as well, to make your unit shiny/glowy/bumpy etc.
5.) upload the model into upspring -- you'll be able to name nodes and set the size and place emission points and stuff like that.
6.) create the animation -- I don't know if there is a tool for this, I never knew of any. Try to look for the animations of a similar unit as an example to start off from. Aside from the creation of the actual model, this is probably the most time intensive step. It helps if you know how to program, but the commands are pretty easy to understand; mostly just translation/rotation of pieces.
7.) create the unit definition -- again, copy/paste from another similar unit and tweak to your hearts desire.
8.) put all the files in the right place -- I'm not sure if the /cheat menu is dynamic or not, but there are commands to /cheat your unit into the game for testing, somewhere in the wiki.
It has been a while, I may be missing some stuff but I believe those are all the basic steps required to make stuff from scratch.
Good Luck!