Not sure if the
B2410 and your
B21 are the same, but it was pretty simple. First, I looked to see which hole the pins were supposed to go through vs other holes in the axle. In my case, there was another hole in the axle outboard of the holes that should have had a pin through them (remember, I had no pins in use at all).
I jacked it up so one wheel was off the ground. Then removed the wheel lugs and the wheel. That exposed the hub. I loosened the bolt that held the hub on the axle and simply slid the hub outward to the next hole. Put the pin in it and tightened the bolt to keep the hub tight on the axle. Then put the wheel and tighten the lugs.
Then switch sides and repeat. The bolt that holds the hub on is the key to making this work. It is designed to wedge the axle against the hub. Fairly simple once you see it in person, but hard to describe.
Hope this helps you.