The goal with any refill modification is to allow the mechanism move like the Pilot G2 does. The Jetstream works a little differently as the point where the spring rests on the G2 is a bit different from the Jetstream. If you look at it up close the Jetstream front is a little longer, so some extra from the back has to be trimmed off. Here is that close up of the fronts:
So from here you need to take off the end piece on the Jetstream:
Next, trim off about 1/4" of an inch with sharp knife. I didn't get a measurement of before, but always trim less because you can always trim off more later and it is nearly impossible to add on more. From here you put back in the end piece. You'll have some of the goo at the end come out and just wipe that off.