This applies to 1st and 2nd Generations of the Solo, 2i2, 2i4, 6i6, 18i8 and 18i20.
Once you have installed the driver go to your computer's Control Panel > Sound option. In the 'Playback' and 'Recording' tabs, make sure the Focusrite/Scarlett option is selected here as the default device. If it isn't already, right click and select 'Set as Default Device'.
Open FL Studio, open the options menu and go to the Audio Settings and set the device to Focusrite USB ASIO. You'll also need to set playback tracking to either Hybrid or Mixer.
You can change the buffer size from the ASIO Control Panel, which you can open by clicking "Show ASIO Panel"
Go to the mixer window (View>Mixer) and click on the master channel. Make sure the output is set to Focusrite (in this case we are using Output 1 and 2).
To receive input from the microphone, go to an insert channel and select the input as the desired microphone/s:
With these settings you can then press Record and Play to record your audio which will appear as a waveform once recorded (note that the 3.2 count in is turned off, which prevents the initial count in):
You can then export this by going to File> Export and selecting the desired format. From here you'll be asked to choose a name for the file as well as it's location. This will then bring up the rendering window where you can choose what you are exporting and other details:
When you are happy with the settings hit start to render.