Before you start, assuming you have the correct factory set-up right now and shifting is smooth:
1. Put your bike on the work stand.
2. Put chain on highest rear gear, ie smallest cog.
2. Put chain on lowest front gear, ie smallest chainring. This will establish the maximum amount of slack.
3. Rotate pedals forward sloooowly and note the clearance between the chain and the small cog on the cassette. The clearance should be 1.0cm or less, but the chain needs to clear the small cog. On my factory built Trek Top Fuel the clearance is about 2-3.0mm. On my Lemond the clearance is about 7.0mm.
4. Anyway, you know where things are set at the start.
5. Install your new cranks.
6. Then drape your chain over the small cog in the rear, through the deraillier and over small chainring in the front. Pull the chain ends together and check the clearance. You should now know the number of links to remove.
Did you know:
1. You very likely need a compact specific front deraillier for smooth shifting.
2. ...to be careful threading the rear deraillier...keep the chain under both tabs.
3. Don't break the chain where the connecting pin is located. Use a new connecting pin, not the factory one you punch through.
4. Put a plastic tie around the face plate in front of the link you're going to break so that you put the chain back on in the same orientation.
5. The new crankset should be installed using a torque wrench, it's best not to guess. These are alloy parts and the cranks need quite a substantial amount of torque to properly install.
Good luck. I just did this; it went well.