The back of the envelopes containing the invites

Invitations

First things first, invitations! We decided to send the invitations via post, to add an element of surprise and excitement for the guests. There are 11 guests, and each one needs a personalized "letter of acceptance". Using Warner Brother's Personalized Hogwarts Acceptance letter and the text from the book as references, I created a template in Photoshop (attached). 
The actual wording can vary according to your needs, it's just a matter of personal choice. You can even print a second page with "books and equipment" if you want, though I did not. For Minerva McGonagall's signature, there were two options I found. I used the longer version I found just by Googling "McGonagall's signature". WB's letter has a shortened version but I preferred having her name written out fully for the guests. I downloaded the Hogwarts crest from a Google search, there are many varying designs you can choose from.
Each child's name will be inserted where necessary and then placed in envelopes.
The most convenient option for printing the letters is in color on normal A4 paper, and I've changed the background color to a slightly off white color to add a parchment feel. I did visit a few craft stores to look for parchment colored paper or envelopes, but to no avail. 

Link to WB's Hogwarts Personalized Acceptance Letter 

On June 2^nd, my little sister will have her 11^th birthday party, and she is insistent on one theme: Harry Potter. TO be honest, I am just as excited as she is; our whole family from ages 20 to 10 has grown up with J.K. Rowling’s magical novels, and I am the most enthusiastic Potter fan of the four of us.Other blogs and web pages have been dedicated to Harry Potter birthday parties, and now I will take it upon myself to create another. I will post photos and updates about the process, hopefully helping any other party planners in organizing a very potter party.

-Sydney, Australia