There is no "super simple" way to do this. I laud you for creating a template for this.

First, I would use a macrobutton field prompt or a text content control for where the address and body of the letter are to be placed. Here is a free Add-In you can download to make such prompts in a document or template: MacroButton Prompt Add-In

You could then use an Advance field just before the prompt for the body.

In setting up your template, especially if you are going to have different margins on the first page, I would recommend you look at How to set up a letter template and perhaps the Letterhead Textboxes and Styles Tutorial.