Make sure your parchment paper or cardstock is the same size as a regular sheet of printer paper.
Save the Hogwarts school crest and the seal to your images in your computer
Paste them into a word document so it looks like the final picture I posted. Make out your letter however you want to. Or you can research the actual acceptance letter and retype it.
Load the parchment or cardstock into your printer and print your document. Congratulations, you have been accepted to Hogwarts School of Witchcraft and Wizadry.