You can use -- to decrement (count down) just as you can use ++ to increment (count up).
Seconds-- ;
Or,
Seconds = Seconds -1 ;
Then use an [u]if-statement[/u] to stop when you get down to zero.
I am totally unsure how to code this, any guidance would be much appreciated.
I suggest you start with a down-counter that simply counts and displays the results. Then you can add the if-statement. Then add the timing. Then add the code to take care of the minutes and seconds, etc. ...Just work on one little thing at a time and "develop" your program. Don't try to write the whole program at once.
The most common way to make a counter that stops at a certain count is to use a [u]for-loop[/u], but the timing makes it a bit more complicated.