In most cases, demos are publicly available, and you can always look at your game and figure out what you did wrong, what happened, and what could've been done better or differently.
Write down your mistakes, conclusions, thoughts, and questions that arise while watching demos as that will help you improve down the line. In the process of analyzing the match, it's almost impossible to keep all the information in your head, with some of it getting distorted or even completely forgotten. But if you make notes, then after watching the game, you can run through the notes and pay attention to important points, making it all absorb a lot better.
Try to find a causal attitude and your own approach to each situation. It's often easy to shrug off the responsibility in a lost 5v2 post-plant situation, where you ran into an enemy, thinking that the round was already won, but you ended up dying first. Surely you can tell yourself that you played relying on the info, and it was your teammates who lost the round, but is it really true?Sometimes it is more difficult to look at the situation self-critically and in detail; reflect: why did you play like that, how did your actions affect the round, and how could you have played better? Did you want to get more kills or did you hope that the team would win the round without you? It is important to pay attention to such things – it shows the consequences of not playing until the end of the round. It can save many rounds in the future.
Remember, you should focus not only on the poor plays, but also on the good things in your game. This is necessary to transfer successful experience from one match to another and consolidate the knowledge. Sometimes you can make several mistakes in one round, but still win. Reflect not only on the moments where you died, but also on the moments where you could've played better, but for some reason didn't. Don't be afraid to make mistakes! If you analyze your game, it always pays off. Even the best of the best sometimes make mistakes, the main thing is to draw the right conclusions.
When watching a pro player's demo, pay attention to how his behavior changes depending on the information he receives. Look at what positions pros take and observe their crosshair placement. Just imagine what you would do in a particular situation and compare it with what a professional player did.
Analyzing and comparing your play with professional players can bring up questions, and a well-posed question is already half of the answer.