You can do it through Microsoft Word or similar word processor.
-Use yellow printer paper
-Setup the page background with light blue lines to imitate a yellow notepad (optional)
-Use mail merge with your excel file of contacts to insert the prospects name at the top, and for the info you’ll print on the envelopes
-Find and download a “handwritten” font to use in your document, or the option I’ve used is go to a site like YourFonts.com and turn your own handwriting into a font 👍🏻
-Setup the printing size in Word for the envelopes you have
If you’re going for the traditional yellow letter you’ll also want to have your font color as red.
One last thing, don’t start your entire print job on the first try. You’ll have to test print a couple of envelopes and letters to make sure everything comes out as it should. After that, you can print as many as you want at once; but, be keeping an eye on them to make sure they keep printing properly.
Good luck man