To process incoming audio to your DAW by connecting external hardware (ie a reverb or compressor unit) to your interface please do the following:
Connect your hardware line output into a jack line input on the Saffire or Scarlett, for example line input 3. Ensure your instrument or microphone (sound source) is correctly connected to one of the analogue inputs, for example input 1.
Open the Mix Control application and route 'Anlg In 1' (or other analogue input if not using input 1) to one of the line outputs for Zero Latency routing, for example Line output 3 (see below). Then connect line output 3 to the line input on your external hardware.
Check that output 3 is set to Silver; you can hold Shift and click on the output number to change it. This means that the output monitor control knob will not affect them and the signal is sent to the external hardware at full level. These settings are circled in the screenshot below:
Once set up you set the gain for the channel that is connected to the audio source first, then set the gain for your external hardware, and finally set the gain for the input from the external hardware.
You will now be able to process your Audio and record it into your DAW as a 'wet' signal by selecting input 3 or use the dry signal by selecting input 1 (or relevant inputs depending on which you are using).
To find out how to use an external effects processor to process already recorded audio, see the article here.