How To Make A Landing Animation? [When hitting ground]

You must use humanoid state type to get the results you want and should look like that: For e.g the local script is inside the player's character

script.Parent:WaitForChild("Humanoid").FreeFalling:Connect(function() --- get function that fires during freefalling
    if script.Parent.Humanoid:GetState() == Enum.HumanoidStateType.Landed then --- check whether the player landed
        print("landed") --- print landed
    end
end)

When testing your game in studio you'll notice that inside the Character (I think) has a LocalScript called animations or Animations (don't remember). What you want to do is copy paste that object and then place it in StarterCharacterScripts. Inside the script you'll notice a bunch of animation id's. Those are the Roblox default animation. All you want to do is make your own animation and change the falling animation id with yours.

PS: If you want a custom one I suggest looking up the properties of Humanoid and some events.