How can (could) you say it another way? "I'm deciding what to say (do) without preparing or planning in advance." The gist of your sentence and mine is that "I" am doing something without any advance preparation or planning. As events occur, I decide the next thing I'll say or do right then. "I am improvising my words (actions) on the spur of the moment." "on the spur of the moment" (idiom) = on impulse; suddenly; without premeditation

It usually refers to when you are unsure of something. For example, you got promoted to a new position and no one told you what you need to do everyday. So if someone asks you what do you do at work. You may answer, I make it up as I go along. Hope that explains it.

as I go along = on the spot He's making it up every second.