First, you have to have a variable call forwarding feature on your line - this is available from your phone company for a monthly fee.
They can tell you exactly how much it will cost - a few dollars / month usually - and they will probably want to bundle it with various other calling features - so be sure of what you are getting.
There may be instructions on the exact procedure in the front of your white pages.
The precise options available vary with the phone company, but, in general, you can set up:
1) forward ALL calls to a particular number (which you can enter)
2) forward calls when your land line is busy
3) forward calls when your land line goes unanswered for a certain number of rings (which you can set)
With variable call forwarding, you can turn those options on and off and change the number to which the call is forwarded whenever you want.
Some companies can also allow you to make some of those changes remotely (i.e. when you are away from your home phone, you can change its forwarding behavior by calling a certain number, entering a PIN number, your phone number and various other keys to indicate what you want to do).
Some phone companies also allow a selective call forwarding - where you can enter 10 or 12 numbers that will be forwarded as you indicate - and all others will ring as usual.
One other thought: if you enter a forward-to number that happens to be long distance, then the charge for that long distance call will show up on your land line phone bill.