Video
Details
Port forwarding is simply telling the router that anything coming on port 80 (default port) to be routed to the OpenSprinkler IP (eg. 192.168.1.22). So in order to port forward you will need to know this IP address which can be found on the LCD by clicking B1. Also, the application is capable of scanning the local network and identifying devices which also shows you the IP.
The OpenSprinkler IP is assigned automatically by the router and can sometimes change which will prevent the port forwarding from working. In order to prevent this, you may setup a static DHCP on the router which assigns the same IP each time to the OpenSprinkler. In order to do this the router needs to be told the device MAC address which can be found on the LCD by clicking B2.
For details on your specific router please consult the router manual and look for "static dhcp" and "port forwarding". The above video demonstrates the most common brands which covers a large majority of use cases.