The FSM says to use their elongated conical condom-like special tool to fit the boots onto the axle shafts. I've been there before with boot guns and KY jelly and W123 axles – broken fingers and busted boots and frustration can rule your world...
...not worth trying to do that during my life span again...
…plan B => build a simple wooden press.
EDIT - please note this wooden press is a "make your life easier" suggestion. It isn't 100% necessary. See comment here =>
W201 annular axle re-boot – much easier than W123 homokinetic axles!
Clips fitted to the boots. Big washer used to spread the load.
All because the lady loves Milk Tray
Finger nails and dignity intact. Take that you rubber bastards!