The discount code field appears on the top right hand side of the checkout page (not to be confused with the cart) and should be entered before clicking the "continue to payment method" button.
If paying with PayPal you can enter the discount code either before or after completing the PayPal express checkout.
Combining discount codes
Generally speaking, multiple discount codes cannot be combined to achieve a higher discount. For example - you cannot enter a 10% off everything and 20% off everything coupon to get a total of 30% off your order.
However, coupons that apply to specific products can be combined if the applicable products have been added to your cart. For example - if you enter a 20% off Photoshop and a 30% off Procreate coupon the discount will be applied to any Photoshop or Procreate product in your cart.