If your battery is going flat when your car sits for a while (it shouldn't) measure the current drain at the battery by measuring the current (amps) between the negative terminal of the battery and the negative (ground) cable.
Is it more than tens of milliamps? Then you have something that's on that shouldn't be or maybe even a short.
Start pulling fuses. Most of them will not change the reading at all, but when you pull one and the reading changes down to a smaller number, you've found the circuit that's on or shorted.
Possible things that might be on but shouldn't be are glove box lights, trunk lights - things like that.
You can do this without a voltmeter - just connect a small light bulb between the negative terminal of the battery and the ground cable. When the bulbs goes dimmer or off, you've found the offending circuit.
Once you've found the offending circuit now you have to chase all the things on that circuit with an ammeter or voltmeter to find the one that's not doing what it's supposed to.