This is hard to explain in written words....and nearly as hard to show. I made sure to include this procedure (with pictures) in my transmission repair book. I'll keep it short and sweet here. If you need more explanation, I suggest getting a copy of the book.
The second gear has a small hole in-between a couple teeth. Use a small drift punch (1/8" or smaller) in this hole. You must align the hole with the hidden key and plunger pin inside where the gear rides on the mainshaft. You should be able to just see the edge of the key as you look at the front of 2nd gear. The key rests in one of the 6 splines.
Rotate the gear until the hole and plunger line up. Press the drift punch in the hole. You should be able to feel the punch move the plunger inward slightly (against a little spring). While doing this, use a small screwdriver to push the key back toward the rear slightly. Remove the punch and keep pushing the key back til it jams in one of the "cut-outs" of the spiral spline.
Now, you should be able to rotate the thrust washer that is located just in front of 2nd gear. When the washer splines line up with the mainshaft splines, the washer will come right off. Now, second gear with it's bushing can come off. Sometimes the bushing will stay there and the gear comes off alone. Just be real careful when removing the bushing to capture the little spring and plunger pin that will suddenly release from behind the bushing and key. They are hard to replace for some reason.
Now all the parts are all out. It's usually a good idea to replace the bushing. Reverse the procedure to install.