Don't wait until the next time you experience negative emotions, start now. Acceptance requires awareness and both these skills require a little bit of practice. If you were participating in a sports event, you probably wouldn't wait until the event to practice. It's the same with Distress Acceptance techniques. You can start by observing and describing your feelings right now, to strengthen your introspection muscles.
You might have wondered why we ask you to log your mood in the daily activities section almost every day. You recovery journey is, at its core, a journey to master your emotions. Describing how you are feeling is a good way to connect with your inner emotions because it forces you to observe. Next time you're making an entry, describe the physical sensations you feel with whatever emotion you are feeling.
Pro tip: When you're experiencing a negative emotion, if you find it hard to observe and accept, write or log your mood. Be as descriptive as possible about what you feel, how you feel it, where you feel it, why you feel that way etc. It'll really help you detach from your emotion, and make it easier to accept.