On this motor, it's not a problem at all. Hook the inner end to the catch on the mainspring hub (that can be the toughest part), then use the key to wind the mainspring into place. There is a hook bent on the outer end of the mainspring that just wraps around one of the crossmembers, so when the mainspring is almost completely wound into the motor, hook the end and finish winding. It helps to wear leather gloves and eye protection because the end of the mainspring can whip around with some force at times. But, other than being careful to avoid the loose end, it is just about that easy.
Now, a Hafner... that's a whole 'nother story! 
