You use a function generator by powering it on and configuring the output signal to your intended shape. This entails connecting the ground and signal leads to your oscilloscope to check the controls. Then you adjust the function generator until you get the appropriate signal and attach the function generator’s ground and signal leads to the device under test’s input and ground terminals. While attaching to ground will typically suffice, in some situations you may need to attach the function generator’s negative lead to the device’s negative input.