Giada lets you record anything coming from your sound card through its live sampler and the main input channel. First configure your device in the Configuration panel, then raise the input volume knob in the main window.
The live sampler works only with sample channels. Add at least one sample channel, then arm it using the channel arm button 
. You can, of course, arm multiple channels. When you are ready, click the audio recording button 
in the main transport or press the end key to enable the sampler: channels with an active input turn blue.
The sequencer starts automatically, since recording audio with the sequencer turned off would not make sense. By default, the sampler works in overdub mode: audio coming from the sound card is continuously added to the existing audio until the sampler is stopped.
Arm the channel and enable the input monitor option in the channel drop-down menu. This lets you hear what you are playing, processed through the plug-in stack, if any. If you do not enable the sampler (using the input recording button or the end key), Giada will not record anything and will instead work as a live effects processor.
You can achieve a similar result by using the stream linker 
, the button located directly below the input meter in the main window. Enable it to hear what you are playing.
Pressing the audio recording button also starts the sequencer. In some cases, however, you may want it to wait and start only when the input audio level reaches a certain threshold. This mode is called record on signal, and you can enable it by clicking the record on signal button 
in the main transport. You can set the desired threshold level in the Configuration panel.