Just throwing my experience into the pot here as I solder these things on, on a daily basis.
Wire the capacitors to the terminal post first.
Twist the remaining capatitor legs togeter and bring them over the side so they lay on the side of the motor casing.
Take a small needle file and file the plating off the case in the area you wish to solder to. Removal of the plating is important and it will be obvious when you are through to the steel casing.Apply a little flux paste to the metalwork.
Tin the legs of the capacitors and the exposed area of the motor casing and solder them onto the tinned casing.
Cut off the surplus capacitor wires to tidy up.
As has been pointed out, you will need a decent hot soldering iron, mine is a 48Watt and even with that, it needs to be running at full heat as the motor casing will sap the heat from the iron quickly so solder it immediately and do it quickly as you don't want to start heating up the plastic motor backplate (if yours has one).