Leveling Up

There are two primary methods of character advancement: Experience Point and Milestone.
In Experience Point systems, throughout your adventure, characters gain experience points (also called "XP") that represents the results of how they have grown, practiced, and learned new things. Accumulating enough experience points results in your character gaining a level which grants new powers and abilities. Once you cross the threshold of XP documented below you gain a level.
On the other hand, the Milestone system has no definitive traceable metric for the characters to keep track of. Rather, the GM decides at critical steps in the adventure, "milestones" that a level has been gained. These can be things like finishing a boss battle, clearing a dungeon, solving a difficult puzzle, or enacting a critical narrative plot point.
However your character gains a level, follow the steps below to apply all of the benefits to your character. Please note: unless your GM specifies otherwise, you do not regain any expended resources (HP, Spell Points, etc) until after completing a Long Rest. So while your HP and Spell Point maximums may increase, you must complete a Long Rest to fully restore your HP or take advantage of new Spell Points.

Step 1: Apply Any Ability Score Increases

As noted in the chart below, some levels (4th, 8th, 12th, 16th, and 20th) allow you to increase your main Ability Scores (also known as ASI). When reaching one of these levels, pick 2 ability scores to increase. If the current value is 17 or higher, it increases by 1. Anything 16 and lower increases by 2. You cannot apply this bonus to any given stat twice at the same level (i.e. when you reach level 4 you cannot increase a Str score of 12 by 4, but you can choose Str at level 4 and again at level 8).
Keep in mind that once you cross an even number your modifier changes and this increase applies retroactively. For example: a character of Con +2 who becomes level 8 and is now Con +3 applies an additional +7 to their HP. This is because HP is calculated using the Con Mod and you had 7 levels where it was +3 but now acts like 7 levels of +4. So you gain those retroactive points as well.

Step 2: Add New Class Features

When you level up, your character can advance to the next level in their current class or take a level in a different class (this is called Multiclassing). Your class will show you what abilities you get at what level. Once you reach that level in that class, make sure to add those abilities to your character sheet. Some abilities you already have also increase in power at higher levels, so be sure to check those as well for any improvements.
Your class also specifies your HP dice. Every level after first you roll this die and add your Con Mod to the value and increase your HP maximum by this amount.

Step 3: New Feats

Every odd numbered level, your character can take another feat or forego taking a new feat and swap an existing feat for another. Make sure when taking a feat that you meet the prerequisites.

Step 4: Theme Benefits

At levels 6, 12, and 18 your Theme grants you additional benefits. Apply these to your character sheet when you meet the requirements.

Step 5: Invest Skill Points

Upon level up, your character gains 5 skill points. You can place these in any skill so long as it would not push that skill point value over your character's level. Don't forget to update you skill bonuses both with skill points and any new Ability Modifiers.

Character Level	Minimum XP	Notes
1	-	Feat, Theme Benefit
2	1,300	-
3	3,300	Feat
4	6,000	Ability Score Increase
5	10,000	Feat
6	15,000	Theme Benefit
7	23,000	Feat
8	34,000	Ability Score Increase
9	50,000	Feat
10	71,000	-
11	105,000	Feat
12	145,000	Ability Score Increase, Theme Benefit
13	210,000	Feat
14	295,000	-
15	425,000	Feat
16	600,000	Ability Score Increase
17	850,000	Feat
18	1,200,000	Ability Score Increase, Theme Benefit
19	1,700,000	Feat
20	2,400,000	-

Multiclassing

On average, players continue in the class they chose at level 1 to gain access to the higher level abilities. Sometimes, however, a player wants to combine multiple abilities from different classes to create a unique combination. this is called "Multiclassing". If you chose to do this, when you level up you apply the "level 1" attributes of the new class to your character. While your character is only ever considered a single level (for example a level 8 character) those levels may be split between different classes (a Level 6 Thaumaturge and a Level 2 Cryomancer).
When you add a level of a different class, you use that class' hit dice to calculate your HP increase and what dice you have in your pool of Hit Dice. You also gain any proficiencies or bonuses to checks, saving throws, and gear the new class grants. You also gain the +2 skill bonus to any skills from any of the classes your character has.
It is also important to read carefully the prerequisites of abilities and feats. Some specify a class level while others specify a character level.
You can take as many levels of as many different classes as you want, but while it might seem tempting to be a dilettante, spreading yourself thin comes with a cost. Since you always start at the ground floor with a new class, it’s easy to end up with a bunch of low-level abilities that can’t compete with the higher-level abilities of a single-class character of the same level.

Re-Spec

Generally speaking, once you have made a decision when leveling, that decision is final. Should a situation arise where you wish to re-spec your character there are 2 very rare items that you can use to do so with your GM's permission.
Draught of Skillful Abandon
This potion removes all skill points currently applied to your character. You gain a pool of skill points equal to your character level x 5. You then immediately re-allocate these skill points.
Elixir of Magical Transfer
This potion removes all levels applied to your character. You then immediately must re-apply the levels of your choice. After consumption, this potion is takes effect the next time you complete a long rest.