From App Launcher, search and select Letter Template, Click New
Add a Name for the template
Choose the Template Type (either a letter or email)
Choose a Source Object
Copy your template text from a source document and paste it into the Template Body field. If formatting doesn't look correct, paste the text into Notepad to strip the formatting and then copy/paste from Notepad into the Template Body field. Adjust formatting using the Rich Text tools.
Click Save
Notice the fields in the template are not the actual API names of the fields to be merged, just placeholders until the next step.