The Ladder is one of the last tools you can unlock in Animal Crossing: New Horizons. It will allow you to climb cliffs and access the upper parts of your deserted island.
Get the Ladder DIY Recipe
You'll need to have played a certain amount of the game in order to unlock this DIY Recipe.
First, you will need to have unlocked the Nook Shop. Once the Shop is built, Tom Nook will ask you to allocate three plots for three houses. Place them anywhere on the island, and he will then give you the FIY Recipe for the Ladder.
Wood x 4
Softwood x 4
Hardwood x 4
Using the Ladder
Using your ladder is very simple. Equip it via your inventory (X button) or via your Tool Wheel (Directional Buttons on the left JoyCon) and stand at either the foot or edge of a cliff.
Press A, and your character will then use the Ladder to climb up or down.
Later on, you will unlock the Town Hall, and Tom Nook will offer to build ramps in order to access the upper parts of the island more easily.
However, you'll need to keep your Ladder with you when you set out on your journey to the mystery islands.