I use the app 'Autodesk sketchbook pro' on a tablet. It costs about £5 (sorry if this is not your currency)You so swag. If i even atempted to do these i would most likely stab myself in the eye (on purpose). What do you use to draw these? Or computer?
But there is a free version -. It doesn't have as many layers( used to erase colour or lines without effecting other parts).
Lots of different effects and brushes that offer control over radius of the brush and opacity.