With the Draw tool selected, click in the timeline and drag to add an event using the track's active clip. The longer you drag, the more times the loop will be repeated (one-shots and Beatmapped tracks do not repeat).

Repetitions in the loop are denoted by the black indentations in the top and bottom borders of the event.

To change the track's active clip, click the Paint Clip Selector button in the track header and choose a new clip from the menu:

When you are ready to continue, click the Next button.