I recommend that everyone familiarize themselves with memorizing a pro game. To that end, I recorded myself memorizing a game yesterday. It took about 40 minutes starting from 1h15m into this video https://www.twitch.tv/videos/260323908 . Expect that memorizing your first game will take longer than that.
You can get many, many pro game records here: http://gokifu.com/ . Pick one at random or a game by a player you like.
First, open a game in your favourite SGF editor. Flip through the game casually, then look closely at the first batch of moves. For beginners, I recommend 5-10 moves at a time. Replay them. Once your move creates a branch, you know you’ve made a mistake. Go back, re-memorize, repeat.
You don’t have to memorize an entire game, either. Usually 100 moves should already give you a good intuitive impression of common opening strategies and a little bit of middle game fighting tactics.
For bonus points, replay the game on a real board now, but rotated by 90 degrees.
Let me know if you’re having any trouble.