It depends on the headset, but you can either connect the headset base between the handset cord (curly cord) and the handset or there is a headset jack on the bottom of the phone as well. Once connected, without any additional programming, you have to pick-up and hand-up your handset to make/take calls. If you do not get dial-tone, you may have to play around with the switches on the base of the headset unit.
If you program a Headset Toggle Button on your phone, you can activate it- telling your phone you are in headset mode, then just press a line button to grab dailtone without lifting the receiver as above. A drop button comes in handy with this option as well so you can hang-up using the drop button.