Farming Simulator 22

First buy a horse stable from the shop. Navigate to the “Others” tab, then “Construction”, then “Animals”, “Horses”, and pick one of the 3 available choices. Once bought and placed, go to the paw symbol by your placed building and select “Open Animal Dialog”. Buy any of the horses and then walk into the fenced in area and get on the horse. Press cn_RT three times so that your horse is now “cantering”. Once cantering continue to ride your horse until the Riding percentage in the top left reaches 100%. Once at 100%, the achievement is yours.

TLDR: Reach 100% Riding with a horse.

Edit: Turns out you actually might have to press one more time once at 100% to Gallop with the horse for the achievement to unlock. If someone could confirm or deny please do.