To set a timer you first need a Timer set-up book. to get it you type in chat "/trigger TimerSetup" (without the "").
On the first page of the book you can set how long the timer will be and see what is it set to now.
On the second page there are different settings for the timer. you can hover the mouse over the setting to see what it does.
On the third page you can start, stop, pause and unpause the timer.

Write %appdata% in the top text bar in your file manager.
Find the .minecraft folder and open it.
Open the saves folder.
In there find the folder of the minecraft world you want this data pack in and open it.
Drag the downloaded zip file in the datapacks folder.