Screens is designed to work when you have a connection to the Internet. However, it is still possible to connect to a computer when you are using an computer-to-computer network, also called an ad-hoc network.
Note: make sure that your Mac is configured to receive incoming connections via Screen Sharing or Remote Management first!
1. Create an computer-to-computer network
2. Get your Mac's IP address for your computer-to-computer network
3. Select your ad-hoc network
4. Create a new screen by hitting the Custom... button
5. Enter your Mac's IP address from Step 2
Note: you can leave port number set to 5900.
6. You should now be able to connect!