Ask your Dom. They are the ultimate judge of whether or not you are being a good sub. Pay attention to their rules, rituals, protocols, etc. If you don’t understand something, ask for a clearer explanation.
Set your limits. Having limits doesn’t make you a bad sub. On the contrary, it means you are looking out for your Dom by not putting them in a bad position when those limits are crossed. It’s one of your responsibilities as a sub.
Don’t let yourself get hurt. Some subs, particularly when bottoming for S&M play, feel that they need to take as much as they can to please their Dom. There’s a difference between pain and injury. If you get injured because you were too stubborn to use a safe word, you could land your Dom in some very real legal trouble in addition to the guilt they would likely feel.
Don’t rely on your Dom to do everything. Do research for yourself. Learning as much as you can about the lifestyle will help you protect yourself from abusers who use BDSM as a cover/defense for their actions. It will help keep you physically safe when undertaking play that carries physical risks since you will understand the risks and be able to help your Dom mitigate them. It also shows that you care about the lifestyle and are enthusiastic about your role.