I can tell you how, but I also advise against it.

1. To get a thumbnail (preview), open File | Properties in the template and
check the box for "Save preview picture" on the Summary tab; resave the
template.

2. There are several reasons that this isn't necessarily a good idea.

   a. Empty templates won't display anything except the shape of the page.
Templates containing text will be unreadable unless the text is very large,
though the general layout of the text will be visible.

   b. Adding a preview adds unreasonably to the file size. This may not be
an issue with the template itself, but it is with all the documents based on
it. When you create a document based on a template with a preview, the
document also has a preview unless you clear the check box. Even if you have
"Prompt for document properties" enabled, you still have to remember to do
this every time you create a new document.

   c. Quite aside from the excessive file size, document previews are not
really helpful. You can try this experiment yourself. Save the same document
with and without a preview, then view them both in the Preview view in File

not very helpful. The one without a preview gives you a scrollable window
into the document content so that you can actually see what the document
contains.