Creation ~ First I created a wheel and axle. Then I cut out room for the axle and wheels in the model. Fit was pretty good first try and wheels spin with very little effort on my hand.

Assembly ~ Press the tapered axels into wheels first, then pop axels into the cutouts on the bottom. You'll see the axels have a flat side you can squeeze through. You must put the wheels on first because they nest into the wheel-wells. 

Tuning ~ Everything should spin free, if it rubs try to adjust the wheels. There's not a whole lot you can do at that scale to make it better but you are welcome to try.