Hopefully you have a ferrell - your brass doughnut- a brass nut with a hole in it and and a gauge. First, slide the brass nut over the copper tube; next slide the ferrell over the copper tube. Now insert the copper tube into the back of the gauge into the the center of the threaded stud. Keep the copper tube securely in the gauge and slide the brass nut up on to the threads - this will pull ferrell into position. Start tightening the brass nut by hand until it starts to snug up. Ensure the copper tube is still inserted all the way into the gauge. Now the important part - start tightening the brass nut down to crush the ferrell against the copper tube - DO NOT OVER TIGHTEN!!! The ferrel will compress and seal the copper tube to the gauge.