The jig might be handy, but I've been assembling the N scale 1025s for 50 years without one, and the 714 is the same design. The secret, gleaned from a magazine article long ago, is to pick the spring up with a knife blade, rather than trying to put it in with tweezers or fingers.
For what it's worth, the 1025s and 714s couple very nicely, so if you want a slightly smaller coupler, you can use the N scale ones. They are available assembled, as #1023.