This applies to the Scarlett 6i6, 18i8 and 18i20 2nd gen devices and all of the Clarett range.
If you have a device which uses Mix Control then please refer to this article.
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, in this example we will use line input 3.
Open the Focusrite Control application and select ‘Line Outputs 3-4’ and then deselect ‘Stereo’ on this tab so that the tab now displays ‘Line Output 3’ (see below). Then click the source selector below and select ‘DAW 3’. Then connect line output 3 to the line input on your external hardware.
In the ‘Device Settings’ tab at the top of your window, ensure that Line Output 3 is not being controlled under ‘Monitor Controls’. If you do not see these options then you will not need to complete this step.
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 in Focusrite Control, see the article here.