I can feed the plug
wire through the boot and trim to length but do not know how to attach the
spring

Reb.... The prong is inserted through the plug insulating outer covering to
engage the inner steel wire. However, just to be sure of a good contact, I
always trim back about a 1/8" of the insulating covering, then fold the exposed
wires back against the circumfrence of the wire, trimming off the excess so
that it conforms with the roundness of the wire.

I then hold the spring terminal alongside the wire to judge where to insert the
prong so that when I flip the spring around in front of the wire, it will be
firmly up against the steel inner wire.

Now, to insert the wire and spring into the rubber boot easily, spray a slight
bit of WD40 on the spring, wire, and the inner portion of the boot where the
wire will be inserted. Just a little bit. That ought to do it.

Note that the large boot is for the spark plug, the smaller boot is to go to
the coil.

Joe (30+ Years With OMC)