(1) Open Inventoria.
(2) Options -> Other -> Advanced Settings...
(3) Check "Allow Express Invoice, Express Accounts, or Copper to synchronize with Inventoria.
(4) Enter a number or password that is hard for others to guess and click OK. Sometimes I use 123456 for testing, but I'm sure you can be more creative than that.
Leave Inventoria running.
(5) Open Copper and log on.
(6) Options -> Sync Apps.
(7) Select "Synchronize this installation of Copper as a client of Inventoria or Copper.
(8) Enter your code again here.
(9) Enter the IP address. Enter 127.0.0.1 if Copper and Inventoria are running on the same machine. (Otherwise, you may need to go back to the other machine and visit www.whatsmyip.org or use another approach to see the IP address of that machine.) Normally, just enter 127.0.0.1.
(10) Enter the port number, which is 1097 for Inventoria.
(11) Click "Retrieve Locations". Then select a location from the drop-down. If you haven't changed the defaults in Inventoria, it will be "Main Warehouse".
(12) Click "Update now" to make sure it works.
(13) Click OK.
From now on, Copper should update the Inventoria stock counts whenever you sell an item.