The Amber mechanic is pretty straight forward. There are some steps which need to be done one after another with a set "waiting timer" (-> debuff) in-between.
You want to stay together as a group for the mechanic, so stay with the commander.
There is an enemy mob called Plague Carrier Abomination, or "abo" / "abom", walking through the arena. Be careful not to attack it unless your commander calls for it and you go for a damage phase. The abo respawns 20s after being killed.
When you do go for the damage phase, you need to kill the abomination. Upon its death, there will be a white field on the ground. It stays for 10s. The squad needs to run through this field to get the wurm attractant, visible as a yellow buff above players’ heads.
With the buff you need to stack up in front of the wurm. The stacking spot will be marked with a red arrow once you have the buff. As soon as 20 people who have the wurm attractant are stacking there, the wurm will disappear into the ground for a moment and eat the players with buff (others without buff will not be eaten).
place reflection skills (e.g. guardian: wall of reflection, mesmer: feedback, ele: swirling winds [focus 4 on air]) around the group while stacking
use skills that grant you stability if husks or veteran wurms are close to the stack to avoid being knocked back
use personal / group heals when stacking to keep people on full health (combo field water + blast finishers, other healing skills); keep using healing skills after you are eaten
You will find yourself inside the wurm's stomach, and you will constantly take damage. There is the Unfortunate Adventurer in the back. It has a harpoon. Players need to interact with the adventurer to pick up the harpoon and shoot at the pyloric valve on the other side, so the group can be spat out again. After you have picked up the harpoon, there is a long cooldown on taking another harpoon, so make sure not to lose it:
do not use weapon swap while holding the harpoon
do not use anything else that would make you get different weapon skills
do not go down, if possible, while holding the harpoon
stop shooting as soon as the valve is destroyed and hold on to your harpoon - you can shoot as often as you like inside the wurm, but you only have 1 shot left after being spat out
When the group is outside again, everyone will have the condition weakness on them. Use skills which remove conditions (e.g. mesmer: nullfield, necro: well of power, ranger: healing spring, engineer: healing turret, thief: signet of agility, etc).
You will have to shoot the harpoon at the Amber wurm. You can get close to the wurm first, so you can stack up for the damage phase more quickly. 20 harpoons are needed to get to the damage phase (the wurm has 20 stacks of slick skin which make the wurm invulnerable; each harpoon removes 1 stack).
target the Amber wurm - do not have any other enemies as target or between you and Amber
be aware of the wurm's attacks when getting close; orange aoe fields will indicate an incoming attack - dodge!
if you are more familiar with the wurm's attacks and are able to recognize them and dodge in time, you can wait with shooting the (last) harpoon(s) until the wurm starts doing a husk-/egg-spit or aoe spit attack
click here to read about the wurm's attacks
After being spat out by the wurm, players will have the upset stomach debuff on them for 1min 30s. It prevents from getting the wurm attractant. As you will still have the debuff after the damage phase, you do not want to go for killing the abo right away - same goes for the case of not getting enough harpoon shots on the wurm for a damage phase.
If, however, you do not have enough people stacking with the buff, so that the wurm does not eat anyone, you will not get the debuff and it's possible to go for the abo again right away.
Use the time waiting to clear the arena and kill veteran wurms/eggs/larvae.
Stop damaging the wurm at ~15%. Slow down damaging a bit earlier, at ~20%, as the wurm will keep taking a bit of damage from conditions and people not paying too much attention.
You will have to wait for all 3 squads on the map to be ready to decapitate their heads before you repeat the mechanic once more for the final damage phase. The commander will tell you when to start.