GWENT: The Witcher Card Game

More information than guide but the requirements here can be split in two:

1. a leader skin
Easiest way is to purchase a 'bundle' in the shop. It costs 1200 meteorites and comes with a leader skin, an avatar, and a board.
Currently there are three packs available to purchase for three different leaders: Northern Realms' Demavend, Scoia'tel's Brouver & Nilfgaard's Emhyr. Emhyr is the only one unlocked from the start so may be the easiest to use for this achievement. Otherwise you have to accrue Reward points and unlock leaders from the Reward Book (the last icon in the bottom-left corner of the main menu).
To equip, go to the Play menu and you will find a 'Change Leader Skin' option there, just make sure you select a deck with a leader you have the skin for.
Meteorites can be earned playing for free but will require some grinding. So keep saving those. If you played regularly in the beta however you will probably find you have enough or close to it (and Reward Points as well if you want to unlock it with another leader, I used Brouver myself).
(Note you can also unlock leader skins in their Leader Tree from the Reward Book, but it will require a fair amount of Reward Points (RP). You may want to keep them for a different achievement instead -Fantastic Five- to unlock a leader for each faction. But the option is there if you want.)

2. an avatar, border and title
Those are unlocked during ranked seasons. If you played the beta you are likely to be able to equip some straight away (I had several unlocked as soon as I started the game). If not, some leveling up will be required but will be achieved as you grind for the leader skin.
If you've played in the beta, your previous profile has all been saved and is already restored so you wouldn't have to worry about that part.
To equip, press the 'hamburger' button on your controller and navigate to the 'Trinkets' tab.

As of feb. ‘19 (Season of Love) they still haven’t activated season reward tree. So for new players to achieve it this way you still need to wait at least one more month.
However as mentioned by In Gyi Yoon, they have activated new reward trees (press RT on the reward tree screen). There in the Year of the Wild Boar, you can use reward points to get them, although you will need to spend 23 RP for the border, plus 16 RP more for the title. Similarly to above it may be worth keeping those for the leaders instead but the option is here.