Bring your pass to the place where it will be redeemed (ferry crossing ticket office, etc.) and show it to the right employee. In the case of hotels, passengers must call in advance to ask about their rules on using the discount.
No, coupons are not interchangeable. They must be used in conjunction with your pass and are a benefit of the Eurail pass. All is explained in the pass guide, along with directions on the use of coupons.
Not all benefits require a coupon. Most merely require you to show your rail pass—though some do require advance reservations.