How would you go about getting this sort of Art Nouveau style rendering digitally?

It's almost flat colours with very light/gradual shading.

I pretty much just stick to some basic cell shading so I'm really not sure on where to go to get to where I want to be.

>>7611282
>>7611335
I'll be very disappointed if these are flat chested girls.

>>7611442
Based fellow Mucha appreciator.