XP to Level & Proficiency Bonus

Ability Score Modifiers

Prerequisites

In order to multiclass into a given class, you must meet the prerequisites, as detailed below.

Experience Points

Your Character Level is the sum of your Class Levels, as mentioned above. A Cleric 6/Fighter 1 would still need to accrue the regular amount of experience to advance to 8th-level before advancing either of their classes.

Hit Points and Hit Dice

When you multiclass, you gain hit points in accordance with Hit Points at Higher Levels (which is to say, you roll the relevant classes Hit Die (or take the average + 1) and add your Constitution modifier). You only gain the listed Hit Points at 1st Level when your Character Level is 1st-level.

You gain the Hit Die for your additional class, which is of the type specified by the class. If this is of a different type than your main class, you need to keep track of them separately. For example (from PHB 163), a Paladin 5/Cleric 5 would have five d10 Hit Dice, and five d8 Hit Dice.
When you take a Short Rest, you may choose which Hit Die to roll in order to recover Hit Points.

Proficiency Bonus

Your Proficiency Bonus is always based on your Character Level. A Warlock 4/Paladin 1 would have a +3 Proficiency Bonus, as is true for any 5th-level character.

Proficiencies

When you gain a level in a class other than your first, you only gain some of that class's stating proficiencies, as outlined below.

Multiclass Spell Slots by Caster Level

Class Features

When you gain a new level in a class, you get its features for that level. However, some class features have additional rules when multiclassing. These are: Channel Divinity, Extra Attack, Unarmored Defense, and Spellcasting

Channel Divinity

If you already have the Channel Divinity class feature, you do not gain additional uses of Channel Divinity by choosing another class with the feature. You only gain additional uses of Channel Divinity when a class specifies that you do, such as attaining 6th-level as a Cleric.
You may use your Channel Divinity feature to use any of the options from either class, however.

Extra Attack

Unless one class's Extra Attack feature explicitly states that you receive multiple extra attacks (such as an 11th-level Fighter), you do not gain further Extra Attacks from receiving this class feature multiple times.
The Warlock's Thirsting Blade Eldritch Invocation is considered an Extra Attack feature, and likewise does not grant additional attacks if you already have the Extra Attack class feature.
As with Thirsting Blade, the Artificer's Arcane Armament feature is considered and Extra Attack feature, and does not grant additional attacks.

Unarmored Defense

If you already have the Unarmored Defense class feature, you can't gain it again from another class.

Spellcasting

Your capacity for spellcasting depends partly on the combined levels of your spellcasting classes. If you only have one spellcasting class, use the rules for that class's spellcasting.
 Spells Known and Prepared. You determine which spells you know, and which spells you have prepared, for each class separately, as though you were a single-classed member of that class. The preparation of spells, and the number of spells you are able to prepare, are constrained as per the relevant class's rules. In addition, a spellcasting focus (such as a Holy Symbol) may only be used to cast spells of the associated class.
 Spell Slots. You determine your effective Caster Level by adding together all of your levels in Bard, Cleric, Sorcerer, and Wizard, adding further ½ your levels (rounded down) in Paladin, Ranger, and Artificer, and finally adding ⅓ your levels (rounded down) in Fighter and Rogue, if you have the Eldritch Knight or Arcane Trickster Archetype. Use this level to consult the table of Multiclass Spell Slots to the left/above.
 Higher-Level Slots. If you have more than one spellcasting class, this table may give you slots for spells higher than the spells you know - again, the spells you know are constrained per the rules for the class, irrespective of your Character Level. If this is the case, you may use those spell slots to cast any of the spells you know, but you do not gain the benefits of casting the spell at a higher level.

Pact Magic

If you have both the Spellcasting and Pact Magic class features, you may use your Spellcasting spell slots to cast spells learned from your Pact Magic, and vice-versa.