Note that you can collect a Monkey Toy in any game mode and once you collect that monkey toy, the game auto saves and you never have to worry about that toy again. When Points Count Mode is available to you, you can hunt for these chapter by chapter if you like. The game will tell you how many Monkeys you've collected for a given chapter at the chapter select screen which is available through Points Count mode. Unfortunately, the monkeys are not numbered. For this reason you may want to consider having a checklist out so you can scratch off Monkeys as you find them. At any rate, here is a video guide (credit to RavenVanHelsing for the Playlist):
For those that prefer a written Guide (credit to Fizzmatix for this Guide): WET Monkey Toy Written Guide.