When in the 'Personalize' stage of the workflow, click the 'Personalize' button next to an opportunity to begin personalizing.
Here, you will see all of the information that is currently associated with an opportunity. There is a list of contact information associated with the opportunity, which you can edit or rearrange at your discretion (for more on this, click here). Additionally, there are personalization fields that have been automatically filled in by the system, or custom personalization fields that still need to be filled in.
Once all of your information is correct, you can hit the 'Submit to Compose' button (or 'Submit for Approval' if you have Approval Workflow turned on) to move that opportunity onto the 'Compose' stage of the Workflow. The next opportunity that needs to be personalized will pop up on your screen.