Unlock Stablemaster from the Knowledge menu.

You can access the Knowledge window by pressing ToggleRecipeInventory.

Stables are used for rearing horses from foals to fully-fledged mounts. Putting a foal and food into the stable will over time raise it to full adulthood.

Access the Stables by pressing Interact while looking at it.

Foals can be found grazing the lands and by interacting with them, you can pick them up and raise them as mounts in a Stable.

Press Interact while facing a Foal to pick it up.

By placing a foal inside a stable, you can raise it to a fully grown mount. This will require food. Horses will do fine with Plant-Fiber.

Place a Foal and Plant Fiber in the Stables inventory and activate it by pressing the Play button.

Most mounts require a saddle before you can ride them.

Unlock Saddle-maker from the Knowledge menu.

Depending on the type of saddle, your mounts maximum speed, stamina drain, or defenses are modified.

Craft a Saddle in the crafting panel of a Saddler's Worktable.

Once you have saddled your mount, you can press Interact to ride it. To dismount, press Dismount.

The contextual menu in the lower right corner of the screen will inform you of the actions you can take while riding.

Heavy and unwieldy, lances are the first choice of weaponry for cavalry.

Unlock Jouster from the Knowledge menu.

Using a lance while on horseback can inflict tremendous damage on opponents when charging them at high speed.

Craft a Lance in the crafting panel of a Blacksmith's Bench.