All games need some progression, whether that’s a town growing, crafting new weapons, getting more gold, or even a relationship changing between players and NPCs. In DnD, Leveling up is the base for all progression. The higher your level, the further you’ve progressed from your humble (or maybe not so humble) beginnings, and it also helps your DM figure out how to challenge you appropriately.
To start your level up, you first need to… level up. Three methods of which will be detailed at the end. But once you level up, you can start by selecting the class you wish to gain a level in, usually your main class. However, if you have a 13 in the ability score required by both your main class and another class you choose, you could gain a level in that class, though multiclassing is a topic for another article.
Once you have chosen the class, start by increasing your Hit Points either by taking the average of or rolling that classes Hit Die, adding your Constitution Modifier to the result, and then adding that total to your Hit Point Maximum. However, you must add at least 1 Hit Point for each level (Player’s Handbook pg. 15, ‘Beyond 1st Level’ this rule was added in later printings of the PHB and retroactively via errata).
Next, determine if your Proficiency Bonus increases. Your proficiency bonus increases by one after every fourth level (including multiclass), starting at +2 at level 1. So if you reach 5th level, your proficiency bonus increases by 1 to +3. This would also be an excellent time to update the bonuses for any skills or Saving Throws you are proficient in, though your class table contains this information.
After that, look at your new level on the class table for the class you chose to gain a level in, and add any features that appear for that level, updating any resources (such as ki or sorcery points, for example) that appear, and if you are a spellcaster, check your Spellcasting or Pact Magic features for spells you learn and update your spells accordingly.
And congratulations! You have finished leveling up! You can do this in any order you prefer as long as you don’t miss a step. Though now comes the problem of gaining a level in the first place, which your DM typically determines at the start of the campaign.
As you resolve encounters, defeat traps, reach certain milestones, or perform other tasks the DM deems available, you gain a varying amount of Experience points spread evenly between all players (Except for absent characters). When you reach a certain amount of experience points as detailed on the table on the referenced pages, you level up.
Milestone Leveling (DMG 261):
When some major event happens -determined by the DM- you level up. Though you could also implement it with Experience as suggested in the referenced page, making certain milestones award several Experience points depending on player level.
Now, of course, you can always homebrew your methods for leveling up, use somebody else’s homebrew, or stick to the standard methods. Each table is different and enjoys different methods. Just because somebody does it one way doesn’t mean you have to do it that same way. One thing that can kill your games quickly is trying too hard to copy somebody word for word. Taking your twists on things is perfectly fine and even the best thing to do in your campaigns. And now, with this knowledge, go and make your games that much better!