• An empty milk bag (or other similarly robust water-tight clear plastic bag about the length of your forearm--apparently milk bags don't exist in the United States)
  
• A small- to medium-sized flashlight (the brighter the better)
  
• A good supply of duct tape
  
• A diving mask
  
• A snorkel
  
• A pair of swimming fins
  
• A body of water at night with fish in it (preferably warm and comfortable)

Procedure:

1. Cut open one end of the milk bag and enclose the flashlight within.
   
2. Let as much air as possible out of the bag and seal up the end with duct tape. You need to make it keep out water, which is harder than it looks. Try folding over the tape and plastic repeatedly. If you have one of those plastic bag sealers that joins two plastic surfaces through heat...then that would probably work better.
   
3. Duct tape the light-bag to one of your forearms. Test to make sure you can turn the flashlight on and off through the bag with your other hand.
   
4. Go to the lake or ocean or whatever you're using and suit up with your swimming gear. Look at the sky. Make sure it's nighttime.
   
5. Turn on the flashlight.
   
6. Dive in and swim around, using your light to guide you. Whoa!
   
7. Look for fish. Be vewee, vewee quiet. Find one that doesn't seem very responsive and that just stays in one place. It's sleeping. Shine your light on it.
   
8. Poke it. If you're lucky, it's a deep sleeper and it won't wake up. Poke it some more. If you're feeling especially empowered, try pinching its tail. Play around with it until it wakes up and swims away. Don't actually whack it. That would be inconsiderate and I imagine fairly difficult to do with any real force underwater anyway.
   
9. Find another fish.

Notes:

• Unless you're extremely good at sealing plastic bags, some water will get in with the flashlight because the water pressure eventually wins out over your fine duct tape craftsmanship. In my experience this probably won't matter, though. I haven't had my flashlight short out yet, and it's only a few volts so I don't think electrocution is any real danger. If you happen to have a waterproof flashlight, of course, then you can bypass the whole milk bag business entirely.
  
• Be careful! Don't go fish whacking alone. Have a friend with you (and one on shore if possible) to make sure you don't die. Water is often very cold at night, so be on the lookout for your friend's lips turning purple. That's when it's time for a break.