Go to the Command Prompt via `Start / Run / CMD`. And use the following syntax,

Add a route

route -p add [Destination] MASK [Mask] [Gateway]

Remove a route

route -p delete [Destination] MASK [Mask] [Gateway]

Example

This would add a route to network 10.1.1.0/24 via your gateway address of 192.168.1.100.

route -p ADD 10.1.1.0 MASK 255.255.255.0 192.168.1.100