Its fairly easy to customize the body of your workflow message in SharePoint designer 2010. Follow the steps mentioned below in this post.
1. Open SP designer 2010. Got to your ‘send email’ action.
2. Right click, properties, it’ll open a window for ‘Send an Email Properties’
3. The last option there is for ‘Body’, click on the right column where it says <!Doctype html> you’ll see a edit option with “…”
4. Click that and it’ll open a string builder window, where you can type a HTML file. It’ll format your workflow mail accordingly.
For some reason the String Builder that comes with SP 2010 is not that easy to edit. I find it easier to create the HTML with VS 2012 or any HTML editor for that matter and then copying the content into the String Buider.
This way you can totally customize the outgoing email that your workflow action sends.