To create a Mail Merge Letter you have to begin by creating a Template. The template, is usually created in another programme such as Microsoft Word all you have to do is select to upload it in to the system. However there is a 'Text Editor' option in the system to allow you to create that template. 

Here's a screen shot of what it will look like...

Once you've clicked on create a new Mailmerge, the next page you will put a name in for your Mailmerge. In the second box, if you have already created a template, then simply select any of those that you'd like to use. 

If you have not already uploaded a template, then you would need to click on the 'Letter Templates' tab to upload a template.

After clicking on Letter Templates, click on 'Create New Mailmerge Template'. This will then allow you to upload a new template. 

To upload a new template, Simply give it a name, description and click on upload file button to upload the template. You can create the template using any of the formats listed on the screen beside the choose file button. 

To make things extremely easy for you, all you have to do is add a series of 'Customisation Tags' (to find the list, just click on the link "For Information on how to personalise your template...") in to it, this will then allow that information to automatically populate in to your template when you are ready to go go go. 

After you have clicked Create you will see an overview screen which will tell you whether all the tags were recognised under the Validation Results. 

So once the template has been created, all you have to do is Create a Mailmerge Campaign and select the relevant template you have uploaded.

Please Note 

Once you have created your mailmerge templates and used them for your mailmerge campaigns, make sure you don't delete them otherwise the contact interactions will be lost.