If you have drawn cards prior to your draw step, they can be ones chosen to be put back using this effect. For example, a cantrip draw or an Instant used during upkeep.

Spells and abilities are resolved one at a time, so if you use multiple Sylvan Libraries in one stack, each will resolve in sequence. You do not get to draw all the cards at once then put them all back at once.

You can return zero, one, or two cards.

This will count as 2 draws for anything that affects "drawn cards".

If you are going to draw cards due to triggered abilities during your draw step, your regular draw resolves first, but you can choose to order the Sylvan Library before or after any other draws.

If you choose to draw two cards, then replace one or more of those draws with some other effect, the rest of Sylvan Library's ability still happens. If you've actually drawn only one card that turn, you must choose that card and either pay 4 life or put it on top of your library. If you haven't actually drawn any cards that turn, the rest of the ability has no effect.

You always perform your normal draw before this ability because the normal draw occurs before anything is placed on the stack.