As you begin upholstery you have to balance learning to correct mistakes against perfectionism.
Ongoing Self-Inspection & Correction
We all make mistakes. Even professionals make mistakes. One of the biggest difference between a profession quality job and an amatuer job is that the amatuers don't correct their mistakes.
On the same vein, correcting your mistakes will improve your skill more than just about anything else. Correcting mistakes is not fun and easy. Sometimes it involves taking a lot of the job apart. However, when you are in the mindset to correct your mistakes, and you actually do correct your mistakes, your awareness increases dramatically. After you've corrected a lot of mistakes you begin to watch the quality of your work as you do it. Many professionals catch their mistakes as they begin to make them, and can make the needed corrections without much loss in time.
Amateurs often either don't see their mistakes (or don't want to see them) or don't care. They just want to get the job finished and don't correct their mistakes. Consequently, their finished projects look like an amateur did it. Here is something you should keep in mind. Correcting your mistakes may increase the time to recover the sofa by a few hours or even a few days. BUT, you will have to live with sofa for years. Do you want those mistakes staring you in the face for years to come?
Even as you are learning to correct your mistakes, avoid perfectionism. If you get trapped in trying to make everything perfect, you never will finish your project and it may become a nightmare. Realize that you will make mistakes, and that is OK. Correct the mistakes that you have made IF you don't have to take apart a lot of your work. Later on, if you continue in upholstery, you will have a higher degree of skill and knowledge so that you will correct more of your mistakes (which teaches you to make less mistakes).
Just do the best you can with your current level of skill, but keep pressing on. Don't let yourself get bogged down or overwhelmed.