Slide the special SealTight nut and ferrule, in that order, at least 3/16" from the end of your flat-cut, burr-free tubing. The slit in the ferrule should face the nut.
Insert the assembly into the receiving port, pushing the tubing into the port until it bottoms out.
Finger tighten the nut until snug. If you are using a wrench tightened fitting, tighten 1/4 - turn past finger tight for proper fitting operation.