How would I make a part face the same direction my Torso is facing?

One of the methods of CFrame lets you create a CFrame positioned at position and looking at point.

CFrame.new(Vector3 position, Vector3 point)

With this in mind, you could just use mouse.Hit.p for the position and the lookVector property of the character's Torso to make sure the part faces the direction of where the player's torso is looking.

sand.CFrame = CFrame.new(mouse.Hit.p,player.Character.Torso.CFrame.lookVector) + Vector3.new(0, 4.8, 0)

I hope my answer helped you. If it did, be sure to accept it.

player = game.Players.LocalPlayer
mouse = player:GetMouse()

mouse.Move:connect(function()
    local target = mouse.Target
    if target.Name == "Grass" then
            print("Spawning sand...")
        else
    print("You can only use sand on grass!")
    end
end)

mouse.KeyDown:connect(function(key)
    local target = mouse.Target
    if key == "c" and target.Name == "Grass" then

        local sand = Instance.new("Part", game.Workspace)
        sand.Name = "Sand"
        sand.Anchored = true
        sand.BrickColor = BrickColor.new("Pastel brown")
        sand.Material = "Sand"
        sand.Size = Vector3.new(8, 10, 2)

    -- Surfaces:

        sand.BackSurface = "Smooth"
        sand.BottomSurface = "Smooth"
        sand.FrontSurface = "Smooth"
        sand.LeftSurface = "Smooth"
        sand.RightSurface = "Smooth"
        sand.TopSurface = "Smooth"

    -- Position:

        sand.CFrame = CFrame.new(mouse.Hit.p) + Vector3.new(0, 4.8, 0)

    -- Rotation:

        sand.Rotation = Vector3.new(player.Character.Torso.Rotation)

    end
end)