This applies to any Scarlett or Saffire that uses Mix Control
To connect your interface to an external effects processor to process recorded audio within your DAW, please do the following:
Connect your effects hardware line output into an line input (jack) of your interface, for example line input 3.
Open the Mix Control application and set one of the line outputs to the output you will be using out of your DAW, for example set 'DAW 3' to Line output 3 (see screenshot below). Then connect the line output you have chosen to the line input of the external hardware.
Check that output 3 is set to Silver in the output level section by shift and clicking on "3" in the output level section (see screenshot below). This means that the monitor Control knob will not affect them and the signal will be outputted at full level to the hardware.
Most DAWs have an 'External Audio Effect' insert allow you to use external processors as plugins. Select the relevant plugin for your DAW and set the 'Audio to" to the output that you are using, for example output 3. Then 'Audio From' to the input that you are using, for example input 3. In Ableton Live the external audio effect device looks like this:
You should then play the audio that you wish to process and set the input gain for the line input according to the signal level.
To find out how to use an external effects processor when tracking, see the article here.