That is a very difficult job, I have tried it on the blower motor of my E32 750, but the cages were so brittle and cracked. On my website below there is a write up under Fixes - Climate systems, a pdf with pics.
Remove your old blower cages by twisting them in opposite directions, one will twist off gradually by twisting back and forth. The other can be removed in the same manner but now holding the opposite bare shaft with a locking plier. The cages will press on the shafts of the new motor with a soft mallet striking the hub area, once it's flush use a pipe over the shaft that fits on the hub.