Pick the range of Pokémon whose numbers you want to learn, check either the "Random" (for getting a randomly generated Pokémon number within the range) or "Next" (for getting the first number in the range if you have just specified one or proceeding to the next number otherwise) radio buttons and press the "Go!" button. This will show you a 'card' with a National Pokédex number.

In "Guess mode", you will get a text input where you should type in the name of what you think the Pokémon is (note that you have to spell the name correctly, although it is case insensitive; that Mr. Mime is only correct if it has a space; and that the Nidorans are put in as "Nidoran male" and "Nidoran female"). Then either press Enter or click the "Guess" button. If you have absolutely no idea what the Pokémon might be, click the "Guess" button without typing anything into the box at all to have the Pokémon shown to you without losing any points.

In "Flashcard mode", you don't get a text input; instead, you should simply mentally visualize which Pokémon corresponds to the given number. When you click the "Flip" button or press Enter, the Pokémon will be shown, and you will be able to indicate whether you got it via the Yes/No buttons on the card (the Yes button will be focused automatically, so if you press Enter again it will count as a yes answer, but if you didn't get it, you can press Tab to select the No button instead and then press Enter again).

In either mode, if you guess correctly, you get a point; if you guess wrong, your streak is reset to zero. If you simply press "Go" again, or typed nothing in Guess mode, then you will skip the Pokémon without losing points.

Recommended use

If you're planning to do this as a conscious effort to learn the numbers and you don't know them already, I recommend putting in some number range to start with and then cycling through it any number of times you feel like in "Next" mode, guessing when you think know the answer and skipping when you don't. Then try to guess all of them in order, and once you've done that you've successfully memorized at least the order itself. Then you can select "Random" and see how well you can connect a number directly to its Pokémon. Your streak will be highlighted whenever it is greater than or equal to the number of Pokémon within the range (if this happens, it generally signifies that you're pretty good at the Pokémon within this range already and might want to consider switching to a new range or, if you're in "Next" mode, switching to "Random" mode). Your streak and score are reset whenever you change the range or switch from "Next" to "Random" or vice versa - this is the best way to keep track of how well you're doing with this particular range.

Of course, if you think you know all the numbers already or if you're an old-timer who knew them back in the day and want to refresh your memory, you can put that to the test by setting the range to whatever you know and then generating random Pokémon and seeing how well you really do...