For some reason I can't visualize how to embroider on a blank t-shirt without taking it apart. I'm sure it must be really simple, but I can't figure it out! Anybody knows?

You gather as much of the non embroidered parts up and clothes pin them out of the way. Either hoop the fabric or use a sticky stabilizer and put it on top of the hoop, then watch carefully so the fabric does not jump back into the embroidery.