Head swapping pixilation issues in photoshop

[TreeAbbott]

I have two photographs and I would like to head swap a few people with face from another photograph. However when I go to do it, the brushing appreas incredibly pixilated. How can I fix this? Photoshop user on windows

[Bojan Živković]

What you mean by "brushing"? I do not see heads on your screenshot. Can you post screenshot of entire window with Layers panel visible?

[c.pfaffenbichler]

What are the Blend Modes of the Brush Tool and the active Layer? 

Please include the Options Bar in the screenshot.