Create an EndNote Web account with EndNote Desktop by doing the following:
After setting up your EndNote account you can access EndNote Web at: http://www.myendnoteweb.com
The first time you sync your library you will be prompted to save a compressed library as a backup. This is highly recommended so that your data is saved if there is a glitch during the sync process.
To run the sync, click the sync icon on the top of the EndNote window
The references in your EndNote Web and EndNote desktop libraries will be synchronized.
You adjust your settings (Edit-->Preferences-->Sync) to automatically sync your libraries every time you make a change.
You can sync to multiple computers. When you make a change to a reference in one library, that change will also be made in all of your other synced libraries. Make sure that the second library is empty when syncing it for the first time. See the video on this page walks you through the process.