Blending textures in GIMP for custom Morrigan race

[Redflyingmonkey]

Hi. So I have a bit of a problem over here. I got the Morrigan follower and player race, from a patreon page, for which the author kindly provided blend masks to use on GIMP, so I could use my own textures, with his ported face textures for Morrigan. Problem is, after blending everything, this happened.

enb2020_5_15_02_38_54.bmp

I then realized his "masks" were made for UNP body, and broke the CBBE textures I use. I could be doing stuff wrong, but I've tried not using the masks and just take my textures and drop them in Morrigan's folder. This was much better, but, of course, I had a pretty bad seam from the face .msn (i believe), being different from the body. Which is logical, the head being a port, it wasn't gonna fit my textures like magic. I can't change the face textures, as it would break Morrigan's face and erase all the details. 

So, my point is, how would I proceed to create my own masks, for CBBE textures, using GIMP 2.10 ? 

Thanks