You need to put a pic up of her tattoo so peeps can make sure the angles, sizes, colour ect will work.... or it will look crap defo lol.
Ive done a celtic design add on for a work colleague before, the thickness of original lines her arms and everything needs considering in the design big time (she will want it looking like 1 complete tattoo not two just stuck together).
composition is everything in a tat
