This tutorial assumes you know the basics of Blender, if you are a complete beginner, then head over to BlenderGuru's YT channel and follow this tutorial series: Blender beginner's tutorial.

Next, download, read and keep handy the documentation, it has all the details you need for the various materials and what they do etc. Referring back to it to read up on what you're doing is the best way to troubleshoot and learn.
Link to documentation: FaceRig documentation. [drive.google.com]

First, model your prop, make your materials and get it looking right, and make sure your prop is 'facing' the right way!

Note: Make sure you're using Cycles Render!





Make a material for each object and name it accordingly.
propname_shadername_objectname



Note: Once you finish modeling and adding materials, keep that file as your raw base, if you later want to tweak the design it's a lot easier at this stage! Save a new copy to continue with. (something like 'myprop2.blend')

The naming is important as if you type it out wrong the importer won't recognize it. (You can however change the material in the importer, so not a disaster if you do it wrong)
Also, if you want an icon for your prop you'll need to play around with the materials to make it look nice, but only the name is important for the importer.
Important note: The only things that will be exported from Blender is the geometry (prop shape) and material names!

Start to apply all your modifiers, mark your seams, unwrap the UV's.
Note: I suggest applying all modifiers except mirror before marking seams, then apply mirror and unwrap. (saves you half the seam-marking job.)

Open up the UV editor, make a new image and save it as a square (i.e. 64x64, 512x512, 1024x1024 etc.) how big depends on the level of detail you need and size of the prop.



If you did not select the desired colours for the UV's images then load them up in an image editing software and tweak the colours, then save them as .png.

Now we're ready to export. Go to File > Export > Collada (Default) (.dae)

Here's the documentation on the Import Wizard:
Import Wizard documentation[drive.google.com]

Importer can be found in your steam library folder, so find your Steam folder and navigate to: Steam\steamapps\common\FaceRig\Bin\Tools\ImportWizardNew\ImportWizard.exe

If you're going to be making a few props I suggest making a folder for it and dropping a shortcut to the wizard there, - you'll be using it a lot.

Starting it up you get this sceen, you want 'Model Importer'



Now browse for the folder that holds your Geometry file and click ok. It should load up all the different objects, listing their materials and shader template.
Note: Make sure 'ImportType' is set to 'Generic Prop' before you import.



Now remember all those UV images I told you to save as a .png? This is where they come into play.
Make sure they are the colour you want then choose each item and plug the corresponding .png into the 'Diffuse' slot.



Now just click 'Import' and you're done! Load up FaceRig and have a look.
(You'll find your prop has a ? for an icon, which I'll address shortly.)

So, we've got the prop, tweaked it to be perfect, now it's time for the little touches.

Remember the materials we named before? Now it's time to use Blender to render us an image we can use for the icon.

Here's a blank background image I made.[i.imgur.com]

Open up your Blender file and to go the materials, first set the colours correctly and then try to emulate the shaders.




Then render the result.
Make sure you have transparency ticked on and the quality ramped up to 100%
Then press F12 to render and F3 to save.
Open up the background image linked above and pop your render result image on top, and voilá, you have an icon!



Save this file as ico_propname.png

Note: FaceRig does not pick up the icon by default, not sure why. Here's a workaround:
Find your Steam folder and navigate to: Steam\steamapps\common\FaceRig\Mod\VP\PC_CustomData\Objects\accesories
Find your prop folder and drop your icon in there.