Large metal objects inhibit your ability to solder because they draw all of the heat away from the point of...well... soldering? Normally you would either have to leave your soldering iron on the piece of metal long enough to heat the entire thing up to about 800 degrees Fahrenheit (which can take a LONG time), or you would settle for a weak joint and end up hot-gluing or epoxying over it in order to make sure that it won't come off.
However, by following these few simple steps, you can create a strong solder joint and not have to worry about the joint breaking.
Soldering iron (preferable)
Step 1: Prep / Getting Solder Onto the Metal
Clamp the metal so that the flame from the propane torch will not melt/burn anything you have lying around
Light the torch and make the flame really low (see picture).
Hold the torch up to the metal and heat it up.
Be sure to move the torch back and forth so that you don't melt the metal itself.
Every once in a while remove the flame and test to see if you can melt solder on the hot metal.
Once the solder begins to melt, heat the metal up for a few more seconds and then remove the flame for good (you can turn off the torch now).
Quickly apply solder to the hot metal, try to cover the entire area you are going to use.
Don't be afraid to use excess solder, solder is your friend =).
You want the solder to look shiny when it cools. If it doesn't look shiny then quickly heat it up with the torch and let it cool without touching it.
Step 2: Solder the Object to the Large Piece of Metal
Prepare your object for soldering, commander.
Tin the leads / wire you are going to solder onto the wire.
Once again, don't be afraid to use a lot of solder; Solder IS your only friend that enjoys getting poked with a soldering iron.
Once tinned, tin the tip on your soldering iron, leave a good glob of solder on the tip.
Swipe your iron tip, glob facing down, over the solder patch on the piece of metal.
You want to get some solder onto the solder on the metal, and make sure that when you finish the solder that you just put on is shiny. If it isn't shiny, torch it until it is and try again.
Now, hold the lead / wire over the new, shiny patch of solder and again swipe your soldering iron over the lead / wire, pushing down slightly. The solder that you recently put onto the patch should melt and merge with the solder on the lead / wire.