Plumbing – Connecting push fit to a 3/8in supply faucet line: NPTF to compression threads

Yes, quite simple.

1. use a Scotchbrite pad to shine up the copper tube so the ferrule ring has a clean, gouge free seating area
2. slide the nut on
3. slide the ferrule ring on
4. push the copper tube into the fitting till it seats (fittings I've used have an internal stop at about 1/4" from ferrule seating area, double-check that the fitting is designed this way)
5. push the ferrule ring into place against the fitting
6. thread the nut on by hand (make sure the tube doesn't back out of the fitting and till nut is hand tight)
7. tighten the nut just till the fitting is leak-free (about 1/2 turn from hand tight)

No plumber's goop, no teflon tape on assembly, the seal is formed by compressing the ferrule ring.

Where most people go wrong is to over-tighten the nut for the ferrule ring. The ferrule ring is trapped between two conical surfaces (nut and fitting) that force the ring to bite into the soft copper which both seals and forms a mechanical connection to keep the tube from backing out. Over-tightening causes the ring to dent the tubing, causing leaks and the inability to disassemble and reassemble the fitting as all the give is taken out of the ferrule ring and it cannot compress any more to seal effectively on the next reassembly.

Since it seems that you already have the lines, I think all you need is a compression adapter fitting like this:

http://www.amazon.com/LASCO-17-6759-4-Inch-Compression-Adapter/dp/B008E5CFC4/

It will screw onto the female 3/8" compression on the drinking water faucet and allow you to connect a 1/4" compression line to it (from the filter).

This is one of the most commonly sold compression adapter at Home Depot, so anyone in plumbing should be able to point you to it (if you're in the US... I'm sure Lowes and others sell it to. Quite common.