1. From the Company menu, choose Prepare Letters with Envelopes.
2. Select Customize Letter Templates.
3. Here you can Create a New Letter from Scratch, alternatively View or Edit Existing Letter Templates.
Once you've made the appropriate selection you will end up in Micorsoft Word where the rest of the process can take place.
When in Microsoft Word click on the Add-Ins tab and you can see a selection of available data fields you can use in the customised letter template.
Here is an article that goes into more detail on a step-by-step process.
Whilst it makes reference to QuickBooks, the process is essentially the same.
Reckon Data Recovery Team