Don't get your hopes up too high, sometimes it really just does not work out so well. And even if it does at first, a little bit of chain stretch will make it problematic.

But, here are some things to keep in mind:

-Use one of the on-line calculators to figure out what the ballpark combos are.
-Get your hands on a couple (or more) chainrings that differ in tooth count as little as possible.
-Get your hands on a couple (or more) cogs that also differ as little as possible.
-Consider filing flats on the axle to give yourself a little more wiggle room.
-Consider judicious filing of your dropouts to give some wiggle room.
-Get a halflink, it doubles your chances of success.

With a halflink and two cogs and two CRs, you have 8 shots at success. With a bit of filing you might bump that up to 16 or 32 chances.

You have a healthy chance of success, but even so it will be frustrating.

Oh yeah, measure the chainstays EXACTLY. And when using the on-line calculators, focus first on those gear combinations that fall dead one exactly. Only resort to the combos near the fringe when you are desperate. If then.

Pray to your gods and good luck.

jim