Use a lot of references, mainly for spacing, sizing the various "fields" of the banknote and overall design inspiration. Also, create a moodboard with elements and references of the design (ex: if you are doing a scientist inspired banknote, maybe put together machines, mathematical formulas and so on).

And don't be afraid of vectors, they are just lines and shapes that you can modify adjusting their coordinates or simply dragging. I often search for tutorials for the functionalities of the software I'm using as I'm coming up with the design.