You still pay all costs for a spell you cast from your library, including additional costs. You may also pay alternative costs.

Magus of the Future doesn't change when you can play lands. You can do so only during your main phase when you have priority and the stack is empty.

The top card of your library is still in your library and not in your hand. You can't discard or suspend cards from the top of your library, for example.

If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you don't reveal the new top card until you finish doing so. This means that if you cast the top card of your library, you can't look at the next one until you're done paying for that spell; and if you play a land from the top of your library, you can't look at the next card until you're done handling any replacement effects that modify how that land enters the battlefield (such as that of Vesuva).

You must follow the normal timing permissions and restrictions of the spells you cast from your library.

Playing a land from the top of your library counts as your land play for the turn. Once you play a land during your turn, you won't be able to play an additional land from the top of your library unless another effect (such as that of Azusa, Lost but Seeking) allows you to.