just got a new stiffening cap

You are right about needing to charge the cap.  It's best to use a 12 volt light bulb because it will tell you when the cap is charged as well.  First, without a ground connection on the cap, apply power to the (+) terminal, direct from battery.   Second, hook up the light bulb between the (-) terminal of the cap, and a good ground using aligator clips or test leads.  The first thing you will notice is the light coming on, that means current is flowing and the cap is charging.  After a while, you'll notice the light getting dimmer, this means the cap is charging and drawing less current.  This will go on for anywhere from 20 seconds up to a minute or two, depending on the light bulb wattage.  Leave the connections on until the light eventually goes out.  This induicates 0 current flow and a charged capacitor.  Now you can remove the light bulb and aligator clips.   To double check the charge state of the cap, remove the (+) terminal on the cap from the battery and measure for 12 volts across the two cap terminals.  If you don't disconnect the (+) terminal you're measuring the car battery voltage as well.  If the cap is charged, reconnect supply to the (+) terminal (which is parallel to the supply voltage to your amp), and connect a good ground and you're done.

Once charged, treat the cap as a car battery, because it is capable of storing power for a long time, even without being connected to a battery.  If the car is ever stored for long periods of time without power fed to the cap, it will eventually dissipate.  Always check to see if the cap is still charged with no battery before reconnectin a battery to it.  If it is not charged, follow above procedure before connecting beforehand.

By the way, the reason this is all necessary is that an uncharged cap is basically a dead short, so hooking power to it and a ground without a resistance in line will short your battery straight to ground.