On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Clan War: Starting the War. You can start this quest in the Sanctuary location during the completion of the main mission named Bright Lights, Flying City.
1) Bounty Board
2) Ellie's workshop
3) Hodunk camp
4) The Holy Spirits
Use the Board , accept the mission and Fast Travel to The Dust. There, speak with Ellie . After the conversation, take two pieces of Dynamite from her garage. Head to the back of the garage and take the Hodunk and Zaford Emblem from the scrap yard. Now you have to move to Hodunk camp  and plant the Dynamite on the pillars of the sign from the above screenshot.
The falling sign will create a passage to the camp, so head inside. Place the Zaford Emblem in the designated place and return to Ellie  to use Fast Travel and get to The Highlands Overlook. Plant another dynamite on the Zaford distillatory  shown in the above screenshot and afterwards put the Hodunk Emblem in its remains. Head into The Holy Spirits pub  beside the distillatory and speak to Mick Zaford to end the mission.