Very early in The Siege of Paris DLC, you will be offered missions in order to help La Résistance--er, I mean the rebels against Charles the Fat. As you complete these missions, your infamy in Francia will increase, entitling you to better upgrades and gear from the rebel quest givers. The highest infamy level is 4, at which point the achievement will pop.
There are six rebel mission quest givers located around Francia, all denoted by a white winged insect emblem. Each mission is very straight-forward and usually amounts to little more than "kill this NPC over here." While I did not take an exact count, it will take approximately two dozen missions to reach infamy level 4.
The rebel quest givers will also allow you to purchase items as well as upgrade support troops that can help you in each quest. I found the melee troops to be kind of useless and so I invested all of my resources into upgrading the ranged infantry (make sure you select archers before accepting a new quest.)
Reaching level 4 is also required for purchasing all of the gear necessary for the "Vendange" achievement.