Need help photoshopping a head onto another body?

[kimchi]

I've tried absolutely everything and I cannot get this to look realistic. I've been trying (for days) to add my sister's head onto another body. Long story short: My sister's birthday is this weekend and we are trying to make this "mock photo" into a card for her, it's a hilarious somewhat inside joke. It would be GREATLY appreciated if someone could put her face onto the other girl's body and make it look real.






For photo number two we would need the head on the right.

 

[Paul]

My size options as malfunctioned this is all i can save (size wise)