How would I make a stamina bar, but not for speed or jump power, but for tools?

Flow charts are helpful in situations like this when you're trying to figure out how a script would work. Here's what I would do:

1. Call a function when the tool is equipped, by using Tool.Equipped:Connect()

2. Somewhere in the function you'll want to have a while true do loop, remember to include a wait, to decrease the server-sided stamina value of the player. Obviously, you'll also want to fire to the client here to update the stamina gui.

3. Inside the loop towards the end you'll want to have an if statement to check if the stamina value has reached zero. If so, you can get the humanoid and use Humanoid:UnequipTools(). And, of course a break when the tool is deselected, either by the player or by running out of stamina.

Example to summarize:

local Player = Tool.Parent.Parent -- tool is inside backpack which is inside player

local Stamina
local RemoteEvent
-- not sure where you have these things so I left them as empty variables, for the sake of the example

local Equipped = false

Tool.Equipped:Connect(function()
    Equipped = true
    -- important things pertaining to what your tool does here
    while true do
        Stamina.Value = Stamina.Value - 1 -- 1 is how much stamina is being subtracted each loop
        if Stamina.Value ~= 0 then
            RemoteEvent:FireClient(Player, Stamina.Value) -- do your gui work
            if Equipped = false then
                break
            end
        else
            Player.Character.Humanoid:UnequipTools()
            break
        end
        wait(1)
    end
end)

Tool.Unequipped:Connect(function()
    Equipped = false
end)