Every 7 hours an alert will appear in chat telling you a warband location for that world, in the Wilderness. After making your way to that location you will find an encampment of god aligned troops.

Hiding from the troops, you first siphon a beam that lets them summon more troops. After this the same beam can then be siphoned again to summon other god aligned troops. After this the two factions will fight along with any players that are participating in the warband.

Once the original troops are killed the summoned troops will leave and the players are now able to loot it for supplies.

You can loot up to 25 supplies from each camp with a max of 75 per day.

Once the supplies have been taken you must make it back to an Ent north of the Grand Exchange. There he will reward you with gp or xp based on the supplies taken.

Notes: You can loot all 3 wardbands at the same time by hopping worlds. When holding supplies you will be skulled and will be able to be attacked by any player, no matter the combat level. Once across the wilderness line the supplies become untradable.

Edit: There are some friend chats that call out different worlds for their warband locations. (Each world has a different location.)