hybris apply global discount via web service

I'm trying to create my custom web service and I want to apply a global discount in the cart and recalculate it. I've created a custom addOn for my OCC call, but I'm struggling with the DTO concept and how to implement my method. Basically, the code I use to add a global discount so far is this:

• I'm getting the session cart


• adding a dummy discount


• 
  

  recalculating via the calculation service

  
  

  applyDiscount(@RequestParam(required = true) Double value) cartService.addGlobalDiscountValue(cartModel, new DiscountValue(“description”, double, value, cartModel.getCurrency().getIsocode())); calculationService.calculateTotals(cartModel, false);

  
  

Where should I implement the DTO? Can I use GlobalDiscountRowDTO ? And how to modify the code to be sure I'm using the best practices?

Thanks!