Grab a 14mm socket-cap screw, 3X10X4mm bearing, M3 washer, and an M3 nut.

Wheel bearing assembled... 

Insert the bearing into the wheel - you'll need a set for the right...

And left.

Grab a thin hub and four short plastic screws.

Make sure it's nice and tight... wibbly wobbly wheels are worrisome!

Spin the bearing around a bit to make sure that it's free and clear.