“Printed Clown Robot tokens” are official printed creature tokens which are both Clowns and Robots. You can use the ones that appear in some Unfinity booster packs, or ones from any other set that happens to include tokens that are both Clowns and Robots.

You’re allowed to hide the tokens anywhere on the battlefield, broadly interpreted as the table where your game is taking place.

The tokens can be hidden under other cards or objects, face up or face down, as long as they aren’t touching each other. No stacks o’ Clowns.

You must hide the tokens while the game is in progress.

You can’t break other rules while hiding the tokens. For example, even if we interpreted your library as part of the battlefield for this purpose, slipping one into the card sleeve of a card in your library during a search effectively marks your deck, and that’s not allowed.

Your opponents may report that they’ve spotted a Robot Clown token even if they can’t literally see it, provided they can identify where it is. That is, you can’t just put your hand over it and claim that can’t see it. Well, you can claim that, and it might be true, but it didn’t work for Entirely Normal Armchair and it won’t work this time either.