I am adding my own guide for this, firstly so I can provide more detail for those new to Pro Evo, and secondly because it glitched for me and I would like to explain how I got round it.
So, this is an achievement you can get in "Become a Legend" mode only. You can choose either to play as an "original player" (a player you create) or an "existing player" (a real-life footballer such as Lionel Messi or whatever). The achievement is achievable in either option.
In this mode, one of the menu options available to you is "Items". Here you can buy and/or equip items to help your player develop. One of the items in the "Shop" is called "Skill Boost". It costs GP500. This is what you need to buy and equip. To earn GP500, you will need to play some games. Please note that GP earned in other modes or even other saves of this mode are not available to you. You must play games with this player to earn money for this player. You get GP40-70 per game depending on the competition.
Once you've earned GP500, you need to check which skills your player has already got (go to Personal Info > My Data and then press RB until to get to the "Player Index" tab). Make a mental note or write them down. Bear in mind if you are playing as Messi or Ronaldo or someone like that there might not be many they don't have. Then, go into the shop and buy "Skill Boost". Choose a skill your player doesn't already possess, and then go into "Equipment" (also in the "Items" menu) and equip it in one of the slots.
Now you need to grind a little bit. It will take 6-7 months for your player to learn this new skill. You can check on this progress by going into Personal Info > My Data and checking the Player Index again. Don't worry though, you don't have to play any games during this time. You can "skip to a selected date" and choose a point after this period. As soon as the date passes that your player has learned this skill, it should pop.
If you are after the rest of the achievements in this mode, you will likely not want to sim all those matches, but keep playing games in order to be World Footballer of the Year and play in the International Cup and all that. I only simmed because it glitched on me the first time round and I was anxious to get it.
Now, here's the problem I mentioned: I did all of the above, as Messi, and while I earned all of the other Become a Legend achievements (except for become Captain, which is for created players only), this did not pop. I checked, and the skill I had picked had indeed been learned, but no achievement. I picked another skill, 6 months later learned it, still no b'doop.
A little concerned, I decided to start my created player's Become a Legend career. I needed to do it to get the Captain achievement anyway. So I started a new game of Become a Legend, created my player, and did all of the above again, playing every game. Skill learned after about 7 months, but still no achievement.
At this point I think it has genuinely glitched on me, so I did a couple of things: I went into the dash and cleared my cache (I'm sure we're all aware of how to do that). Then, I popped a memory stick into my 360 and booted up Pro Evo, choosing the stick as the storage device. This effectively started a fresh new copy of all of my system data, preferences etc, for the whole game. After getting the game set up again, I started a new Become a Legend career. I chose an existing player (Francesco Totti in this case), and after listening to the manager ramble on, started playing games. I earned my GP500, chose a skill (Jumping volley), and simmed to a date 7 months in the future. To my relief, achievement popped.
So, If this does not pop for anyone as expected, like it did for me, I recommend doing that. Clear your cache and start an entirely fresh new set of Pro Evo save files on a memory stick.
I hope that makes everything clear, please comment if you still have issues. Thanks and good luck!