A few notes since I don't speak in this tutorial :

1. It's a 10 min video, download and unzip it.

2. You'll need basic knowledge of Blender especially because I realised that the screencast keys addon doesn't work properly, it doesn't display all the keys I'm using (for example near the end where I align the vertices of the mouth control shape in edit mode, I used S-Y-0 and it displays only the S :/ ).

3. The Copy Attributes Addon that I'm using comes already with Blender, you don't have to download and install it. Just go to Preferences and enable it. It's not required but it makes things a hundred times easier. I use it to copy paste the constraints from one bone to another. First you select the bones that don't have the constraint and last the bone that has it.

4. The rig I'm using for this tutorial is Rigify, but you can follow the same steps and create a mouth control for other rigs that don't have one.

5. My Excella model also uses this Mouth Control, you can check it out https://smutba.se/project/29950/

UPDATE : Sometimes you can get weird results with the top lips. This is caused by the Floor constraints. In that case use Pose Space (not World Space that I used) and you might also have to change the axis from Z to Y, depending on your lip bones.

PM me at https://twitter.com/tvitone1 for more help if you need to.