Ok I have a 2 year old Superstar 40 trainer... Along with a set of Fokker Dr.1 tailfeathers... Yiu see where this is going? How do I get the tailfeathers off the trainer and off the remains of the Fokker.. To put the Dr. 1 tailgroup onto the trainer?

As you do this, you'll see why it's called "Kit Bashing"

Get a hobby knife, a few razor blades, some sandpaper, a few misc hand tools....

Do I need to continue?

Fact of the matter is, you need to cut the old ones off as neatly as possible, sand the new ones smooth (To remove any leftover fuse from the last plane) and epoxy them on.

No tricks, you just gotta have the grapes to get in there and start hacking

After you glue the new tail feathers on, run some weed eater line around for braces. It's light, it's scale (for WW I) and you might not get a really good glue joint. Drill holes in the stab and fin tips and run the line around in a big circle. You can drill through the lower fueslage or clamp the line under a screw head on the bottom. Fix it with a little thick CA or Goop.