I mostly drew the shapes over a sketch with the pencil tool with smoothing turned most of  the way up. Some of the shapes needed more precision so I used the pen tool. Both are really different to use on the iPad, and get very natural feeling with just a little practice. 

I created the sketch in Adobe Fresco, as there are some great guide tools to quickly sketch out ideas, but with some room to be expressive and abstract. 

I also watched this great tutorial by Kendall Plant to start off with the basics and to get some inspiration in abstract minimalism. It's not easy for me.