I don't know all the rules for Fifth Edition, but the system is flexible enough that we usually find a fair ruling at the table, even when we're lost. Over time, I've come around to a certain way of doing things regarding mechanical rulings and table etiquette, so here's a big useless list for you. Some of these aren't even different from the official rules at all, but let's just clarify our approach.
- All official WoTC classes and races are legal. Monstrous races and Unearthed Arcana subject to approval.
- All alignments legal. Evil alignments subject to approval.
- Custom backgrounds are legal where appropriate. This means two skill proficiencies and mix-match of two language or tool proficiencies.
- The Lucky feat is banned.
- Players are encouraged to make characters who have a reason to be on the team and have a reason to go on the adventure.
- Generate ability scores by rolling 4d6, drop the lowest. After rolling, players have the option of using standard array (15, 14, 13, 12, 10, 8). Rolls must be made in front of everyone.
- Level one characters start with equivalent hit points of a level two character. When characters reach level two, they don't gain hit points. From level three onward, they gain hit points as normal.
- If a character is being made as a replacement after character death or retirement, the new character can't be the same class as the old character.
- Replacement characters start at the same level as the rest of the party.
- We use milestone leveling instead of XP. Major quests or multiple smaller quests result in level ups at appropriate junctions. Players can expect to level up at least every three sessions.
- There may be rare instances where a character levels up before others for story-appropriate reasons, but usually everyone will be the same level, including absent players.
- Players may choose to take average or roll for hit points at each level. Rolls must be made in front of everyone
- Multiclassing is allowed, but players are encouraged to provide story-appropriate reasons and methods for their characters to multiclass..
- When going from level three to level four, players are allowed to change their archetype if they're unhappy with their character. This retcon is the only amnesty available for character choices. Past this point, or for changes of class or race, players will need to retire their character and make a replacement. Retired characters become NPCs.
- DM inspiration may be used to make any player (including the DM) reroll any single D20.
- Inspiration is awarded at the start of each session.
- Inspiration expires at the end of each session.
- When combat is on a grid, diagonal movement works differently. The first square moved may be diagonal, but any other movement must be standard.
- We don't use flanking bonuses unless an ability or feature specifically states otherwise.
- Other creatures do not count as cover for defense purposes. The only exceptions are if an ability or feature specifically states otherwise, or if players use their action to provide and receive cover. "I, John the Baker, use my action to huddle on the ground as Gallen throws his soft, shitty body over me, using his action to provide cover."
- Enemies of the same type usually act on the same initiative.
- Some enemies will be minions. A minion dies as soon as it takes any damage. When a minion is affected by something that results in half damage on a successful saving throw, it instead takes no damage on a successful saving throw.
- Drinking a potion is a bonus action, but pouring a potion down an unconscious character's gullet is an action. You cannot force feed your your magic juice to a conscious character.
- You can hot-swap your equipped items at the start of your turn as your free object interaction. Stow your shield and club to draw your greatsword — no problem! But that means you have your greatsword out until the start of your next round at least.
- An item can be passed between two characters by using both their free object interactions. Receiving player decides whether they hold the item or stow the item.
- When making a weapon attack, players can exploit a story-appropriate weaknesses to make a called shot against a creature. Called shots are made at disadvantage and can only be made when disadvantage is not already in play from any other source. Successful hits deal damage as normal, but inflict disadvantage on the creature's first weapon attack on their next turn, much like Vicious Mockery.
- Characters can use the hand holding their arcane focus as their somatic component, so you can hold both a wand and sword while doing your mojo.
- Any character proficient in arcana can cast a spell from any spell scroll, but if the spell is not on your class' spell list, you must succeed an ability check determined by the appropriate spellcasting ability. The DC equals 10 + the spell’s level.
- Wizards don't need to roll to copy spells.
- We use passive insight, which works in a similar way to passive perception.
- When rolling perception or insight checks, any roll below 10 reverts to your passive perception or insight score.
- When helping another character with a skill check to provide advantage, the character providing help must be proficient in the skill.
- Sometimes skill checks will be based on different ability modifiers than suggested by the character sheet.
- Sometimes we resolve checks with a group check, where a majority or half of the party need to pass a check. Natural 1s count as two failures. Natural 20s count as two successes.
- Some skill checks are only allowed for characters proficient in the skill.
- All dice to be rolled visibly on table — reroll any that fall off the table.
- Reroll any cocked dice.
- Reroll any dice you didn't intentionally roll.
- Swearing, drinking, smoking are all OK, but slurs are not OK.
- Sexual content is discouraged and is resolved by fading to black.
- In general, if it's not something you'd feel comfortable saying in the lunch room at work, don't say it at the table.