From the lobby, either create a new game or join one from the list. A game can optionally have a join password (🔒 on its card) and a cap on how many characters each player may bring. The creator (or an admin) can tune advanced settings from the ⚙ gear next to the title on the character-creation screen - things like hardcore/gentle death rules, loot/monster density, and XP split.
Once everyone who's joining has made at least one character, the creator hits "Start Game." That's one-way: after starting, no one new can join, so make sure your party is in before that button gets clicked. Everyone who's already in can always get back into a game they've joined from the lobby list.
The mode is picked at game creation and can't be changed afterward.
Each character has attack and defend (combat rolls), HP/max HP and SP/max SP (spell points), speed (how far you can move per turn), HP/SP regen, and a per-damage-type damage reduction (Bludgeon, Slash, Pierce, Fire, Cold, Lightning, Force, Poison, Necrotic, Holy - can be negative, meaning a weakness). Stats come from your class and race and are set once when the character is created.
Click a revealed floor tile to walk there by the shortest path; orthogonal and diagonal steps both cost one tile of movement, up to your speed. You can't cut across a corner that's boxed in by walls on both sides, and you can't move through or onto a tile a monster occupies (you can pass through a tile another player's character is standing on, but not end your move there). Movement is spent from your move action first, then - once that's used up - from your general action instead, so you're never fully stuck if you want to keep moving.
Each turn gives you one move action, one general action, and one bonus action; the general action can stand in for either of the other two. Attacking, casting a spell, and picking up/equipping items all use the general action; opening a door or drinking a potion uses the bonus action (falling back to general if bonus is already spent).
To attack, target a monster within your weapon's range. A d20 roll needs to beat
10 - your attack + their defend to hit; a hit rolls the weapon or spell's damage dice,
reduced by the target's damage resistance for that damage type. Hovering a monster shows the "to hit"
number before you commit. Turn order is by speed, and monsters take their turn automatically once
every player has acted.