I do not have aphantasia, but a painting technique I follow could work for someone with aphantasia. It’s more about pattern-recognition than imagining something beforehand.

I pretty much start by following whims. I pick some colors and do what feels right. Only after a bit do I look again at the work and think, “Huh, that looks like an alien” or “That kind of looks like a tree,” etc. It’s kind of like making my own Rorschach test - making sense of the noise on a canvas. Then, I start accentuating the pattern I see until others can see it, too. In the end, it’s a unique piece that looks like I planned it all along.

I’m not sure if that sounds up your alley, but I figured I’d share just in case.