Marriott certificates are obtained either by having the Chase Marriott credit card which would give you a certificate annually or by completing a Marriott promotion. These certificates let you redeem for a category 1-5 hotel anywhere in the world that has availability and they have a 6 month expiration.
Making a Certificate Reservation
To see if you have a certificate to use, go to your account activity online and you will see the certificate and the expiration date
Next find a Category 1-5 hotel that you would like to redeem for. For a list of all Cat 1-5 hotels click here.
Next you want to start the process as if you were making an award reservation. Search the hotel and select one day or however many days you have certificates for. You must also select use reward points.
Select the hotel and your room type making sure it is a Category 5 or less
On the reservation page you will see that the free night certificate will be attached
All you need to do is click book now and your reservation will be made and an email will be sent with your reservation confirmation. No points will be deducted as you are using your free night certificate.
Extending Your Certificate
Now these hotel certificates expire after 6 months but you may be able to extend them. You will have to call the reservation line to do this. If the agent you get says that they are unwilling to do so, I would hang up and call again. If you are an elite you can call the elite reservation line at 1-800-399-4229 (you must enter Marriott # to confirm status) or you can call the regular reservation line at 1-888-236-2427. You can then ask them to extend the certificate which essentially means that they cancel this certificate and reissue another certificate with a 6 month expiration date.