As mentioned in the Strategies for Sinners article, I use an improved clinch or a pitzen to attach the Fas-snap to the line.

A fly can move freely on the Fas-snap (assuming you use the correct size Fas-snap) so a loop knot to attach it is pretty much redundant.

NOTE: You will probably need to use forceps or needle nose pliers to put on and take off flies. Flies won't come off of a Fas-snap, sometimes even when you want them to. When you balance it out, that's probably a good thing.