The tiny belts are pressed into toothed grooves on the bottom of the sliders. It's hard to tension and force it back into the groove but it is possible. I know it's less tight than it was when I got it (they probably have a fixture at the factory for specifically doing this) but it was good enough to get me back up and running.

"Just check that the filament can feed freely before printing" is shit advice from them, because you can't see beyond one or two layers into a spool!