If you are unable to complete your ballot on your own, it is permissible to get assistance. There will be a space on the form to indicate the name of the person who assisted you to sign and verify their role. This may be on the form or on the envelope.
If your ballot does not have a section for this specifically, you should contact your election official for instructions. Go to the Election Official Directory. Choose your State and Region to find the contact information of your local election official.