how to make a seprated part of a object Okay, so I have this 

and I can cut it so when I 3d print it I can take off the head and have something in side of it like an Arduino Uno and other stuff
Go to Edit mode.

Select the vertices or faces of the part you want to separate and press 'P'.

This opens the 'Separate' menu, where you can choose 'Selection'.

That's all. Now you will have that part as a separate object.