I've forgot how to cut a face and merge onto another person

[rich2005]

yeah, forgot about that

Using the gmic plugin http://www.gmic.eu using the Blend (Seamless) filter.

Works like:
https://i.imgur.com/m64pD7G.jpg
1. Make and copy selection of replacement
2. Paste as new layer over existing.
3. Make any adjustments - size / position / orientation.

https://i.imgur.com/sL40wEP.jpg
4. Apply the Gmic filter with 'outer fading'
5. Gives this but do not expect wonders, all depends on the original images.