There isn't 1 specific right or wrong way to send out coupons. This is highly dependent on the store owner and their preferences. A few ideas for coupon distribution include:
Send coupons in the order notification or registration emails for returning customers.
Send coupons as part of your marketing emails or newsletters
Place coupon codes on your store homepage to advertise them
Give coupons to your affiliates to use in promoting your sites
Place coupon codes on a business card or invoice.