Here are the steps.
1. Accept Flashissue if you have not already.
2. Click on Flashissue right in your email
5. You can easily flip between your email and flashissue. Simply click on inbox to return to your mail, and flashissue to return to your newsletter (it will save what you have done so far). So it's easy to add content throughout a week or month as ideas come to you.
6. You can search online for information to add to your newsletter.
There are tons of other features, but I suggest you just play around with it and see what you can do!