Each wheel assembly will require one wheel shell, two bearings and one precision shim:
Push a wheel shell over a bearing:
Then, place another bearing with a precision shim on top of it:
And push the remaining open side of that wheel on it:
If the precision shim would prevent you from pushing the bearings well in place, then you can use a hex driver or something fine (tooth pick etc) to reposition it a little bit, so that the bearings can be pushed well in place.
Do this for all 48 wheels
If you have belt-driven Y axis, then do this also for the 4 small wheels (with the small wheel shells, the small bearings and the small precision shims)