Cones and light
Crouch to hide. On a keyboard, hold Shift to draw vision cones. A controller has a vision-cone toggle in the same view.
Inside a cone, in clear light, you are seen. Dim light and some brush count as lightly obscured, and hiding there asks for a Stealth check. Magical darkness, fog, and deep shadow are heavily obscured. A creature without a clear view does not simply walk up and spot you.
Put out torches and candles. A cantrip that strikes the flame, or water thrown on it, puts out a lot of open fires.
The opening attack
Start the fight with a hidden attack from outside the cones. Creatures who had not noticed you can be surprised, and a surprised creature skips its first turn. One party member standing in plain sight starts an ordinary fight. Some bosses cannot be surprised. The examine sheet lists that passive. Read it before you spend the potion.
Pass Without Trace adds 10 to Stealth checks for the whole party while you hold concentration. Shadowheart starts as a Trickery cleric and can prepare it. Rangers and druids can learn it too. Medium armour is fine under that bonus. The check is doing the work.
Minor Illusion is a cantrip. Place the sound where you want the group to walk, outside their starting cones, then open with the hidden attack or an area spell.
After the first round
A rogue hides again with Cunning Action. That is a bonus action, so a missed shot can end back in cover.
An Assassin's hit against a surprised creature is a critical hit. They also roll with advantage against creatures that have not taken a turn yet.
A Gloom Stalker adds 3 to initiative. On the first turn, Dread Ambusher is an extra attack. Umbral Sight makes a Gloom Stalker invisible to creatures that are using darkvision to see in the dark.
The rounds after the ambush belong to elevation and doorways. That is the Tactician page. The trader version of a cone, one character talking and another behind the counter, is the early-gold page.
Official site: baldursgate3.game.