I just use an irig, a 1/4" jack cable, and an instrument of my choice. Typically, you have the option when you first turn it on, to choose vocals, synth/piano, drums and if you swipe enough, guitar/bass. Typically, you won't hear anything till you put the "monitor" switch on.
My first recordings sounded so bad, but I'm gettting a lot better with it. I'm beginning to love garage band.
Typically, it's hard to get a good sound right out of the amp simulator. I tend to tweak the EQ on the amp simulator, and then I EQ the recorded track, and I do this to every track I make. Then I do a final EQ on the entire track with the FX function.
I'm sure the dude will explain it a lot better. Bit I mostly wanted to say don't be too discouraged if the tone isn't very great straight out of the amp Sim. You can tweak it post production
Also, you will want a good set of head phones with a long cable if you use irig. And the irig is only 20$
