Insert the top of the PEX above the top in the fitting. Slide the tip of the PEX tubing which has the crimp ring or cinch clamp on it about the end on the fitting that you choose to screwed onto the copper tubing. Drive it on so far as https://offshore-accitent04567.bloggactif.com/31368255/the-5-second-trick-for-1-2-copper-90-elbow