What is the process to Signing a AMI ?

AMI binary content (i.e. the disk content) is immutable, so once the AMI is created then that content can't be changed: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/modify-ami.html

A limited set of attributes of an AMI can be changed after it is created - see the API docs for ModifyImageAttribute for details.