Use the following method: Take your father's shotgun, go out and run around in your lawn that hasn't been cut for 4 years untill out of the blue, a Mewtwo appears and then shoot it until it's dead. Loot the corpse, and you're finished.
Please further educate yourself in regard to the English language. It's either two Mews, or a Mew. No asking two questions for the price of one, be content with the normal discount or stick to asking one question.
Catch two Mews. Then turn them upside-down. The blood in their head would implode into themselves and they will explode altogether. If the resulting goo comes back together then congratulations, you've got a Mewtwo. If not, then you're out of luck. You can only do this once, you know.
Note: You can't get a Mewthree this way.