It is easy enough to add twenty routes with the -ip route- command under config mode. Before doing that, check any possible summarizations etc.
How many interfaces does the router in question have? If you have only two, you may be able to simplify the routing table considerably, if you have twenty this will be more difficult.
Also remember to enable classless ip routing through the -ip classless- global command.
You do not need to activate ip routes. If your interfaces are correctly configured it will work straight away.
Regards,
Leo