From A forum post by Fungus Mungus at G Mod.com

"Well, after a lot of thought, I figured it out. I will post here so that other people can benefit from it.

It turns out that you need to take off the tension adjustment knob on the base. This allows the base to flex a little more so that it can be slid more easily into tracks on the bottom of the chair. It still isn't easy though. Also, you may want to take the two end caps off of the base as they will probably come off when you start pounding. They are held in by an allen-head set screw.

To make this easier, you should get yourself 2 18" reversable bar clamps (I got 2 Craftsman) clamps that worked out really well. You don't technically need them as I was able to get one chair together without them, but it is MUCH easier if you use these.

Flip the chair seat over and put in on a stable surface with the back facing away from you.

Reverse one of the clamps so that it acts as a spreader. Then put the spreader in between the rails towards the front of the seat...about an inch in front of the front-most pair of screw holes. Start ratcheting the spreader until it spreads the seat apart and the fabric is nice and tight. You probably couldn't go too far unless you have gorilla hands. This widens the rails enough so that the next step (sliding the base in place) is a little easier.

To put on the base, you will need to use a good, solid rubber mallet. Position the base towards the back of the chair, making sure the spot where the adjustment handle screws in faces towards the back. There is room to get the front part of the base into the seat rails when coming in from the back of the chair.

Then use the rubber mallet to hit the base from the back to slide the base forward, making sure to keep downward pressure on the base so it doesn't pop up out of the rails. Alternate from the left to the right side ever 3rd stroke or so. You will need to really pound to do this...I worked up quite a sweat. After a minute or so of pounding, you will have the holes lined up so you can put the screws in. Chances are that the screws won't quite line up...they tend to be off-center on the sides. This is because the base is canted inwards a bit to allow it to be slide into the base. I took a another bar clamp and clamped the rail to the base enough so that the screw holes lined up and was able to get all the screws in.

I was able to do all 6 chairs this way...all in all, it took me a couple of hours, but that figures in the time it took me to puzzle all this out.

Oh, and under NO CIRCUMSTANCES should you take off the screws that hold the fabric in place on the tops/bottoms of the chair...these will be a real pain to get back in. I know because one of the chairs had been taken almost completely apart. This took up about 1/2 hour of fussing with until I could get the fabric back into place.

Hope this helps someone out."