If you could post a couple pictures, that would really help too.
Probably all you need to do is switch the saddles. The seatpost for your new bike stays with your new bike, but you take off the new saddle and put your old one on instead.
Usually you only need one tool. (an allen wrench) But we'd need to see what you've got to be sure.
Here's a very good overview by Sheldon Brown. Further down the page he talks about the clamps and such holding the saddle to the seatpost. http://sheldonbrown.com/saddles.html
I'm gonna go check around for one with more pictures, I'll be right back.
Edit: well, I'm not finding good pictures. Phooey.
Basic idea, look at the assembly where your saddle rails meet the seat post. Probably there are two pieces of metal (like a clamshell) holding the rails between them. There is probably a bolt holding those two pieces of metal together and making them pinch the rails.
Loosen that bolt (probably with hexagonal allen wrench) until you can turn the top piece to right angles with the bottom piece. Now it looks like an "x" instead of a clamshell.
The rails are free, and you can just lift the saddle off the bottom piece of metal. (the bottom of the clamshell)
Now you have a naked seat post!
(congratulations!)
Put the replacement saddle on the bottom clamshell, making sure the rails are both in the little grooves or whatever (if the bottom clamshell has grooves). Turn the top clamshell so it lines up over the bottom one again, and tighten the bolt so they come together and hold tightly to the rails.
Loosening the same bolt just a little will let you adjust the fore-aft and tilt of the saddle. Sheldon Brown's site (the link above) covers that pretty well, but if you have any trouble come back here! Lots of good folks on TE who are a great help on all kinds of subjects!
Last edited by KnottedYet; 04-23-2007 at 08:33 AM.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson