The package came with the white plastic piece, am I to use this with copper pipe?
A picture of the process would be very helpful. Thanks...

The white piece is meant to be used with PEX tubing.

As far as the cap goes, make sure you have a clean and straight cut and then simply push the cap on with a slight twisting motion until you feel it bottom out and fully seat.

The white plastic piece may be the tool for removing the cap, do as Widdershins suggested clean the pipe and install the cap if you need to remove it use the plastic piece, Thanks.

John