Both the process of creating an EC2 instance and launching an AMI are same. In both the process you create an Instance.
EC2 Instance is where you select the already created AMI these are by default provided by Amazon.
AMI can be your manually created machines or AMI created from snapshot for recovery.
Both the process are same.
For creating and launching EC2 Instance take a look at Step 1.
For launching an AMI take a look at Step 5 in the article mentioned below.