If I can do it anyone can.

Call me if you get stuck and I can talk you through it.
Here's how I do it...
1) Take the 2 cap head screws that hold rear bracket about half way out and slide the silver staple holding part of the base all the way out of the tool.
--Careful the pusher that slides the staples forward will probably fall out.
2) Look at the rear of the silver base assembly you just took out of the tool and you'll see a little "finger" projection down inside the rear part of the silver base -- slide a loop end of the spring over the little finger projection.
3) Push the spring under the pully at the front of the silver base and pull (stretch it out to get a good hold on it. Keep tension on the spring and wrap up and over top of the pully.
4) Take the staple pusher -black metal piece that fell out when you took the base apart and orient it so the little finger projection is towards the rear of the silver base assembly.
5) Keeping tension on the spring, put the free loop over the finger projection on the pusher. It's going want to pull the pusher forward and out the end of the base assembly so keep a little tension on the pusher and spring with your thumb.
6)While maintaining the tension slide the silver base assembly back into the tool once you've got it started in so the pusher can't rise off it's rail and flip out, close the base assembly and tighten down the 2 screws you loosened in step one.
7)Smile and have a beer, maybe even watch the Colts play some ball your a Duo-fast repair man now.
Hope this helps. Keep in mind the newer tackers have a silver base. If yours is older it may have a black base but still the same procedure.
Chuck
P.S. It's going out in the A.M. my mail picked up already.