Connect the USB / Firewire Interface to the appropriate port on your PC. Plug in your guitar using a standard 1/4-inch jack cable, and power up the computer. Ensure all the gain and volume controls on your interface are turned to zero, then launch the emulator software.
Activate the input channel of the effects software, and gradually increase the input gain. Touch the strings lightly and watch the input level monitor for any spikes. Once the level fills up most of the green section, without tipping over into the red, you can turn on the monitoring channel. Gradually increase the output volume until it is at an appropriate level.
Connect the audio output of your PC's sound card to the instrument input of your amplifier. Alternatively, if your amplifier has an effects loop, connect your guitar to the amplifier input as you normally would, then connect the effects loop "Send" to your PC's input and "Return" to the PC's output.
Experiment with different effects chains. Altering the order in which the effects units appear in the chain; it can have a drastic effect on the sound. Save your favorite effect setups, so they can be recalled later.