First, you should have an excel spreadsheet with the names and whatever other data you want to use. It should look something like this.
I've named this file names_and_numbers.xlsx.
Next, start word and open the Mail Merge Manager.
A small window (called the Mail Merge Manager) will pop up. We just need to work through the steps on it.
Under #1, select Form Letters from the Create New dropdown choice.
Under #2, click "Get List" and select "Open Data Source" and then pick your excel file. For me, it will be names_and_numbers.xlsx.
You may get a warning like below, just click OK.
You will then be prompted to pick which sheet and which cells, just take the default.
You'll now have the data from the spreadsheet available to your word document. The Mail Merge Manager should look something like this:
Now, go to your document and write your letter. Wherever you want customized data from the spreadsheet to appear, just click and drag the placeholders into the document.
After you drag the placeholder, your document should look like this.
Finish your document dragging placeholders wherever you want them.
When your document is finished, you just need to run the merge. You can send the documents directly to the printer or make a new word document with all the personalized letters. I usually make a new file, just to check that things actually look how I think they should. If it's ok, you can then just send this new document to the printer. To run the merge, go to #6 in the Mail Merge Manager and click the Merge to New Document button.
A new document will open with a new page for each customized letter. You can save or print it.