Score the ends of the cells with a small screwdriver so the solder has a surface it can grip onto more easily (like a large # pattern but with more lines etc).
Use a solder flux 'pen' to put some flux on the ends of the cells, tin the end of my 80W weller by wiping the hot tip on a wet sponge pad then adding some solder to the tip (enough to put onto the end of the cell for a battery bar to attach to).
Using a Deans battery jig, hold the battery bar in place with some pliers, add some more solder to the tip of the iron and push the battery bar against the cell using the iron.
If the iron is lower wattage you can tin the battery bars, but my thermo-nuclear iron can do the end of a battery bar to the cell in less than 1 second.
Picture wasn't to actually show the soldering but you can still just about see it.
The motors are basically the same but I use a smaller variable iron at work, set to 350 degC with a 4mm? chisel bit to achieve the following;
Always try to clean the surfaces before and I always use a Flux Dispensing Pen
to put some flux on any surface before soldering, you wouldn't beieve how much difference it can make!