What I did:
1. Get your face texture, and make sure it's 128x128 & a png.
2. Copy any rbxm of any other included face. Make sure you rename it to the same as your png. (like "rblxface.rbxm" & "rblxface.png")
3. Edit it, changing the line for the texture. I used VS Code for this, but notepad should work. Change the name of the face's texture to your texture's name (like "Annoyed.png" -> "rblxface.png". The specific line is <Content name="Texture"><url>rbxasset://../../../shareddata/charcustom/faces/{YOUR PNG NAME HERE}</url></Content>. Don't change the path, just the image's filename. Your PNG file must be one word (just like "rblxface.png")
4. Put both your rbxm and png in the shareddata/charcustom/faces directory like all the other faces. Also create your desc for it, (like "rblxface_desc.txt").
It should then work.