Perhaps I haven't read enough yet. But suppose a monster is casting Ray of Frost. The damage depends on the spell-caster's level. So how do we get the level from the monster's stats? The number of dice for its hit points? But the die type is different for different monsters. Or do we use its challenge number?
Challenge is the closest equivalent to level that monsters have.
A monster that can cast spells will typically have its caster level, spell attack bonus, and spell DC listed in its traits.
iserith wrote:
prestokeys wrote:
iserith wrote:
prestokeys wrote:
The MM p. 10 has sections on Innate Spellcasting and Spellcasting.
The first says spells are cast at the lowest possible level and to use the CR for level, if one is needed.
The second is more along the lines of a typical PC spellcaster.
The free Basic Rules DMG (2) has the same thing on p. 5.
Not sure if this helps. Turning NPCs or monsters into spellcasters could be fairly abitrary, I guess. A goblin has a natural 10 for intelligence - perhaps he is a wizard because he has an 11 or 12? That would give him a DC 11 for spells and +3 on attacks? With 2d6 for hit points, perhaps he can cast 2 1st level spells, or he might be really good and have 3d6 for hit points and cast 1 2nd level spell, too. I haven't seen any RAW for this - just what I would do.
You can also check out Surfrider's blog, as he has an entire blog section dedicated to Monsters with spells. But I'm pretty sure I directed you there once before in the past 
As mentioned, use the CR for the monster to determine its effective caster level. It's the same way as they calculate Proficiency bonus. (This might trip up anyone who was used to 3E, where it would have been based on hit dice.)
Edit: As iserith mentions, NPCs who are actual spellcasters use their actual level for determining the effects of their cantrips and other spells.
I had understood that challenge rating was only used for innate spellcasting creatures, not for creatures with the Spellcasting class feature.
INNATE SPELLCASTING
A monster with the innate ability to cast spells has the Innate Spellcasting special trait. Unless noted otherwise, an innate spell of 1st level or higher is always cast at its lowest possible level and can't be cast at a higher level. If a monster has a cantrip where its level matters and no level is given, use the monster's challenge rating.
An innate spell can have special rules or restrictions. For example, a drow mage can innately cast the levitate spell, but the spell has a "self only" restriction, which means that the spell affects only the drow mage.
A monster's innate spells can't be swapped out with other spells. If a monster's innate spells don't require attack rolls, no attack bonus is given for them.
SPELLCASTING
A monster with the Spellcasting class feature has a spellcaster level and spell slots, which it uses to cast its spells of 1st level and higher (as explained in the Player's Handbook). The spellcaster level is also used for any cantrips included in the feature.
The monster has a list of spells known or prepared from a particular class. The list might also include spells from a feature in that class, such as the Divine Domain feature of the cleric or the Druid Circle feature of the druid.
A monster can cast a spell from its list at a higher level if it has the spell slot to do so. For example, a drow mage with the 3rd-level lightning bolt spell can cast it as a 5th-level spell by using one of its 5th-level spell slots.
You can change the spells that a monster knows or has prepared, replacing any spell on a monster's spell list with a different spell of the same level and from the same class list. If you do so, you might cause the monster to be a greater or lesser threat than suggested by its challenge rating.
The short answer is: wait till the DMG comes out in a month. It will have the exact details on how to make an NPC monster with class levels.
Impatience requires a little reverse engineering and guesswork base on the Priest, Mage, Berserker, Assassin, Guard, and a couple others. Generally they are Level -3 as CR. They all use d8 hit dice, regardless of class, but low hit dice classes are usually given a lower con and vice versa. Stats are bought very ineficeintly with a lower pool of points (assuming standard human rules). Most of the class features are stripped. Mage and Priest do not have any domains or traditions, assassin does not have cunning action, berserker has reckless attack but not rage, etc.
I would say based on those, make the character as if you are really bad at making characters and then set the CR at level minus 3. Skip all class features except extra attack (at level 5), spellcasting, armor and weapon proficiencies, and if they are not spellcasters then maybe one class defining ability (sneak attack, reckless attack, etc.) Fighters have awesome proficiencies so they get nothing.