That's wise advice: get to know your phone! Open the back beforehand, note the battery voltage (usually printed on it), see if the terminals are clearly marked "+" and "-" etc.

If charging the battery directly (i.e., remove it from the phone and tape wires to the +/-, touch it while charging. If it's uncomfortably warm to the touch, there is too much voltage being applied. Disconnect immediately. Once it cools down, you may still be able to charge in five or ten second bursts, but be aware that the battery is in danger of damage.

Aside: I should add some small alligator clips to my kits. Wire is easy to find; clips are hard.