When Myr Custodian enters, scry 2. Then each opponent may scry 1. (To scry X, a player looks at the top X cards of their library, then puts any number of them on the bottom and the rest on top in any order.)
"Myr will go where the work is. We always have. We always will."n—Urtet, Remnant of Memnarch
As the enters-the battlefield ability resolves, first you scry 2. Then each opponent in turn order chooses whether or not to scry 1. Those who do (likely all of them) look at the top card of their library at the same time, then they decide in turn order where their card goes. Each opponent will know the choices of previous players in turn order before making their own choices.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.