It’s super easy! Here are the steps:
1) Remove the handset cord from your telephone and plug it into the S10/S11/S12 handset jack
2) Plug one end of the short cord into the S10/S11 telephone line jack and the other end into your telephone's handset jack
3) Plug the AC power supply into the S10/S11/S12 power supply jack and the wall
4) Plug the headset cord into the S10/S11/S12 headset jack
5) Put on the headset. Press the headset/handset switch and pick up the phone's handset
6) Slide the configuration switch at the left side of the amplifier until you hear a clear dial tone
Note: that for most phones, the configuration setting should be 1 or 4.