How to use analysis

Can someone give me a simple example from my last game on how to use the analysis by the engine? Mind you, I don't ask you to do the work for me...I just don't know how to approach this. For instance, on my move 21 it says I blundered because I should have doubled my rooks. But I simply don't understand why I should have done that and should have done that now (and not on another move).

So basically my question is: can someone point me in the right direction on how to analyse my games, based on this example?

Hey ruud, actually I haven't found any game of you playing on chess.com. Your rating tells me that you played only blitz or bullet. Analysing your games can bei very helpful but it is a matter of fact that you won't the see the best moves in short time frames. To improve you have to think about your moves. When you then are still playing the wrong moves, ask yourself what was your plan, your idea to do this. Maybe, in daily games, write it on a paper. Afterwords, by analysing the match you can compare your ideas with the moves the computer tells you. I think, like this you'll be able to understand the analysing method slightly better. If you then still have any questions, just write me a message.