Instructions
Search dark caves, grassy areas and ponds for frogs. You can find the Otton Frog in the first area you enter when the game begins. Search the "Graniny Gorki South" area later in the game to find the Poison Dart Frog.
Press "L2" and select the Mousetrap (if you have one) to set a trap for the frog. Set the trap once you see the frog, and wait for it to become trapped. If you don't have a trap, use "R2" to choose a weapon and then hold "R1" and press "X" to use it. You can shoot the frog with a tranquilizer dart to capture it alive, or use the knife to kill it.
Approach the captured frog to collect it. The frog will turn into an item that you'll pick up when you get close to it.
Go into your inventory to use your captured frog. The Otton Frog provides the biggest stamina increase initially, while Tree Frogs do not increase your stamina very much at first. However, if you eat a lot of frogs early in the game, you'll get a greater stamina increase when you capture and eat frogs later in the game.