This applies to the Scarlett 1st, 2nd, and 3rd gen, Saffire, Clarett, Forte and Red interfaces

Re-amping is the process of taking a dry, direct recorded guitar signal and passing this through a guitar amplifier. All current Focusrite interfaces have at least one 'instrument' input so that you can connect a guitar/bass directly and record the dry signal into your DAW. Alternatively, you can use a DI box with a mic or line input to record the dry signal.

This works best when you use an interface with more than 2 line outputs. This means you won't need to sacrifice having speakers connected for monitoring while performing the steps below. If you have a Scarlett Solo, 2i2, 18i8 (1st and 2nd Gen) or an iTrack Solo you will need to disconnect your speakers while re-amping. 

If you are using a Scarlett Solo/iTrack Solo, then you will need to disconnect your speakers and use an RCA to 1/4-inch jack cable in order to connect to the re-amp box.

Note: It's not possible to monitor while re-amping with the Solo or 2i2 since the headphone output mirrors the line outputs, so you would just hear the dry signal through one side of the headphones and nothing else.

Setup

In your DAW, set the output of the track you have your dry signal recorded to an output NOT being used by your speakers (e.g. output 3).

If you're using an interface that uses Scarlett Mix Control, Saffire Mix Control or Focusrite Control, assign 'DAW 3' to the physical output you will be using.

Ensure you don't have any Direct Monitoring/Low Latency Monitoring enabled and that this output is set to full-scale (not controlled by the software volume dial). In Scarlett/Saffire Mix Control, this is accomplished by holding shift & clicking the box on the right side corresponding to that output to turn it grey.

In Focusrite Control, this setting is defined on the Device Settings page.

Connect this output to your re-amp box to take the signal from a 'balanced line' to an 'unbalanced instrument' level (the level your amplifier is designed for), then connect the output of the re-amp box to your guitar amplifier.

Position a microphone to record the amplifier and connect this to an available input on your interface (screenshots use input 1 as the example).

Finally, in your DAW, create a new track with the correct input assigned. Ensure this track is sent to your speaker output and NOT to the output that's connected to your amp via the re-amp box to avoid a feedback loop.

Now press record on this track - the dry signal should be sent through a line output on your interface, through the re-amp box and into the amp, which will then be recorded by the microphone onto your new track.

There are some helpful tutorials showing how to record both a dry and re-amped signal at the same time in this video series featuring a Clarett 2Pre: 

youtube.com/playlist?list=PLvKUXEQ5tcx50JjCGJTrpt-NTREZHCpbs