[spree-user] Checkout page customization

I have to customize coupon code on the checkout page. We have Apply button
in front of the Coupon Code text field. I need to get that coupon code and
apply on order and update the
Order Total section on the same page accordingly. I think this can be
possible but how should I add this in Checkout controller for updating my
order depending upon the coupon code and this will be ajax call.

It will be great help if you suggest me something on this.

Thanks