Play a GOTV demo.
Play a demo straight from in the game.
To do this you must have console enabled in your game settings.
Once you have this enabled to bring down the console you must hit the tilde on your keyboard it looks like this "~".
Then begin typing "playdemo", if you have downloaded your GOTV and unzipped it in the proper spot you will see the file pop up. Navigate to the one you want to watch and hit enter. This will automatically start up the demo for you and begin playing it.
To use the ingame play functions you can enable this by pressing "SHIFT + F2", which gives you a bit faster functionality to your demos.
You can skip rounds, fast forward through warmup countdowns, or even watch some key important parts in super slow mow.
Play a demo from a third party program
You can also use a third party program such as CSGO DEMOS MANAGER, this is a great tool for gathering much more information about your demos as well as being able to watch specific highlights or lowlights of any player in a GOTV match. It also gives you functionality in being able to flag specific players or leave notes on specific demos that you want to come back to later. It also analyses and shows you detailed stats of each demo you add to it and lets you mark each demo in diff colours indicated if you have watched them or not.