You could try battling at the Battle Chateau near the Day Care! If you want a real challenge, try battling the elite four! If you want, you could just battle strangers you find in routes.
Step 2: Sell Anything At Shops!
You could sell your items and medicine at the stores in the pokemon center! Or you could try finding a pokemaniac that would take stuff for expensive prices.
Step 3: Work At Hotel Richissime!
Hotel Richissime is located in Lumiose City! The longer/more you work there, the more money you'll receive (if that's not true then you have to be "more stylish" by being allowed to enter Boutique Couture) Just be careful, if you make just 1 mistake during work, you'll receive less money!
Step 4: Update?
Those are all the ways I know how to make Pokemon money but if I find any new ways, I'll update!