how do i make a timer using textlabel startergui?

StarterPlayerScripts

local Player = game.Players.LocalPlayer
local Label = Player.PlayerGui:WaitForChild("ScreenGui").TextLabel
local InGame = 300

script.Parent.Parent.Started.Changed:Connect(function()
    if script.Parent.Parent.Started.Value == true then
        for i=300,1,-1 do
            InGame = InGame-1
            if InGame < 10 then
                Label.Text = "0:0"..tostring(InGame)
            else if InGame < 60 then
                Label.Text = "0:"..tostring(InGame)
            else if InGame >= 60 then   
                        local minutes = math.floor(InGame/60)
                        local sec = InGame-minutes*60
                        if sec < 10  then
                            Label.Text = tostring(minutes)..":0"..tostring(sec)
                        else
                            Label.Text = tostring(minutes)..":"..tostring(sec)
                        end
                    end
                end
            end
            wait(1)
        end
    end
end)

The first half of the while loop will be formatting the time to look like an actual clock, with zeroes that fill in spaces that aren't being used. If the time was 4 minutes and 5 seconds, it would look like 04:05.

You'll need to figure out whether the number of seconds left is a single digit or a double digit. You can do this by determining whether or not it's greater than 9.

local Label = script.Parent
local Minutes = 5
local Seconds = 0

while true do
    if Seconds > 9 then
        if Minutes > 9 then
            Label.Text = Minutes .. ":" .. Seconds
        else
            Label.Text = "0" .. Minutes .. ":" .. Seconds
        end
    else

    end
end

Let me explain what lines 6-11 do. Line 6 checks to see whether or not the number of seconds is a single digit number. If it is, then on line 7 we need to figure out if the number of minutes is a single digit or a double digit number. Line 8 will run if the number of seconds and the number of minutes is a double digit number. We want the clock to have 4 digits, divided by a colon in the middle at all times, but since we have two numbers that are double digits, we can just concatenate the strings without adding any placeholder zeroes. Line 10 runs if the seconds are double digits, but not the minutes, so it places a zero before the number of minutes.

local Label = script.Parent
local Minutes = 5
local Seconds = 0

while true do
    if Seconds > 9 then
        if Minutes > 9 then
            Label.Text = Minutes .. ":" .. Seconds
        else
            Label.Text = "0" .. Minutes .. ":" .. Seconds
        end
    else
        if Minutes > 9 then
            Label.Text = Minutes .. ":0" .. Seconds
        else
            Label.Text = "0" .. Minutes .. ":0" .. Seconds
        end
    end
end

Lines 13-17 do almost the exact same thing, except we know that the number of seconds is a single digit, meaning we just have to add a zero before the seconds.

Now that the formatting is done, all we have to do is actually subtract seconds and minutes so it counts down.

local Label = script.Parent
local Minutes = 5
local Seconds = 0

while true do
    if Seconds > 9 then
        if Minutes > 9 then
            Label.Text = Minutes .. ":" .. Seconds
        else
            Label.Text = "0" .. Minutes .. ":" .. Seconds
        end
    else
        if Minutes > 9 then
            Label.Text = Minutes .. ":0" .. Seconds
        else
            Label.Text = "0" .. Minutes .. ":0" .. Seconds
        end
    end
end

if Seconds > 0 then
    Seconds = Seconds - 1
else
    if Minutes > 0 then
        Minutes = Minutes - 1
        Seconds = 59
    else
        break
    end
end

wait(1)

The way a clock works is it looks at the seconds first, sees if it can subtract a second, and subtracts a second if it can.

That's what line 21 is doing. If the number of seconds is greater than zero, then subtract 1 second.

If it IS zero, then run lines 24-29. Those lines will look at the minutes and make sure that we can subtract a minute. If we can, subtract a minute and set the seconds to 59. If not, that means the seconds is 0 and the minutes is 0, which means the timer has finished, and we can break.

And obviously, we have to wait 1 second for every second that is going to be subtracted.

I know there was another answer, but the person who posted it didn't go into detail about it.