This is an easy achievement, though I think it is still rare on Xbox (under 10% as of March 2019) because not a lot of people know how to get it. Go to MY TEAM, and use your d-pad to select rewards in the MY TEAM menu (I think it is in the bottom right). Then choose a tier - if this is your first time here (likely) you will mostly likely be choosing emerald. If you have at least 3 tokens, which you can earn from various MY TEAM activities, then you can unlock your first player (which you choose from a list) and POP goes the achievement. It is that simple. If you do not have at least 3 tokens, do an activity in MY TEAM that gives you tokens (see below) and then come back and unlock.
Note that at the start there are 10 redeems per tier, so it will take awhile to unlock each tier, but if you play a lot of MY TEAM you can unlock a lot of tokens. For instance, play DOMINATION and you can unlock some tokens there, etc.
Happy Achievement Hunting.