This Achievement is unlocked by getting 1 skill to Level 3. After you start the game, you will talk to Emily on the Wharf after your vision, and you will get to choose your field of study (it doesn't matter which of the three you choose between Diplomat, Detective, and Occultist). Once you pick one, all 5 skills in that "tree" will become Level 1. From here you will want to pick one of these skills and specifically put all points you ever earn into that one skill. You will notice you have points you can put into your skills to level them up. It takes 3 points to get a skill to Level 1, 4 points to get to Level 2, and roughly 10-12 points to get to Level 3. Then hold the X? (forgot the button, it shows on screen though) button to Validate your choice and make your point usage final. At the end of a Quest you will receive 4 points to put towards your skills. So it'll go like this if you follow my lead:
5 skills at Level 1 initially, pick one of them and put any remaining points into it.
Quest 1 Complete, put all 4 Points into the Skill, if it wasn't Level 2 before, it will be now.
Quest 2 Complete, put all 4 points into the Skill
Quest 3 Complete, put all 4 points into the Skill (I unlocked it here, but you can continue with the final Quest's 4 points)
Quest 4 Complete, put the final 4 points into the Skill if you for some reason haven't hit Level 3 yet.
You can also upgrade your skills by using them in conversations along the way, but doing so will likely use up Effort Points so make sure to use any Royal Honey you find to refill your points. Though as I've shown above, you can get 1 skill to Level 3 without worrying about this at all, as long as you know what to say, your skills are honestly pointless.