Turn Inside Out

Because face-down creatures don't have a name, they can't have the same name as any other creature, even another face-down creature.
If a double-faced card is manifested, it will be put onto the battlefield face down. While face down, it can't transform. If the front face of the card is a creature card, you can turn it face up by paying its mana cost. If you do, its front face will be up.
Because the permanent is on the battlefield both before and after it's turned face up, turning a permanent face up doesn't cause any enters-the-battlefield abilities to trigger.
If a face-down creature loses its abilities, it can't be turned face up with a disguise or morph ability because it will no longer have that ability (or the associated cost) once face up.
You must ensure that your face-down spells and permanents can be easily differentiated from each other. You're not allowed to mix up the cards that represent them on the battlefield to confuse other players. The order in which they entered should remain clear, as well as what ability caused them to be face down. (This includes manifest, disguise, cloak, morph, and a few older effects that turn cards face down.) Common methods for doing this include using markers or dice, or simply placing them in order on the battlefield.
To manifest dread, look at the top two cards of your library. Manifest one (by putting it onto the battlefield face down) and put the other into your graveyard. The card you put onto the battlefield becomes a 2/2 face-down creature with no name, mana cost, creature types, or abilities. It's colorless and has a mana value of 0. Other effects that apply to the permanent can still grant it any characteristics it doesn't have or change the characteristics it does have.
If a manifested creature would have disguise or morph if it were face up, you may also turn it face up by paying its disguise or morph cost, as appropriate.
If something tries to turn a face-down instant or sorcery card on the battlefield face up, reveal that card to show all players it's an instant or sorcery card. The permanent remains on the battlefield face down. Abilities that trigger when a permanent turns face up won't trigger, because even though you revealed the card, it never turned face up.
A permanent that turns face up or face down changes characteristics but is otherwise the same permanent. Spells and abilities that were targeting that permanent and Auras and Equipment that were attached to that permanent aren't affected unless the new characteristics of the object change the legality of those targets or attachments.
Unlike a face-down creature that was cast using a disguise or morph ability, a manifested creature may still be turned face up after it loses its abilities if it's a creature card.
Some cards in the Duskmourn release have abilities that trigger "whenever you manifest dread." In circumstances where you are instructed to manifest dread but can't perform some or all of the steps of manifesting dread (probably because your library has one or fewer cards in it), these abilities will still trigger.