To earn this achievement, you’ll first need to have at least one faction which is hostile to your own faction or clan. You can then try to sneak into one of their settlements by interacting with it, then selecting “Disguise yourself and sneak through the gate”. There will be a specific percentage indicating the chances of you performing this successfully, and you can increase this by improving your Roguery skill. The higher the skill, the higher the chance you’ll have to infiltrate the town.
Once you’re inside a hostile town, all you need to do is complete one of the “issues” within it. These are the quests which are visible when you press ALT by default on keyboard. You’ll see a bunch of exclamation marks which indicate the presence of a potential quest. You can then speak with one of the people marked and offer to help them with their issue. These will usually be relatively simple tasks, such as providing them with materials at a reduced cost so that they can game the local economy and cheat one of their rival merchants. You may need to sneak into a town more than once while you collect the necessary goods from elsewhere.