1. Make sure your computers are conntected through LAN (through Router if you want to play online)
2. Install the game on both computers (If you want to play on closed Bnet you'll need 1 pair of cd-keys of EACH computer)
3. Update to 1.10 on both (only if you want to play online)
4. Start the game and setup a D2 Server, then join the server with your other computer or simply play online with both computers
Be more specific