I love making a bunch of the same card design for invitations, or to send out for the holidays. Having made some pretty elaborate ones, I can vouch that the advice “keep it simple” applies here too. After your 50th one, you will thank me!
Here is an idea for an invitation that works up quickly, is elegant in its simplicity, and offers lots of opportunity for customization. Take a sheet of white or cream-colored drawing paper 18 x 24 inches (45.7 x 61 cm). It should be sturdy, but not stiff or card stock since you will be folding it several times. Tear off one sheet and fold it in thirds, lengthwise. Cut along each fold so that you have three long strips.
Take one strip, fold it in half, then fold into quarters in the opposite direction so that you have a basic accordion-folded “book”. Now stamp an image on the top – this will be the “cover” of your “book”. It will be 6 x 6 inches square. Then you can stamp on the inside pages.
Here I’ve stamped the cover with Queen’s Herald:
Then I stamped an inside page with Queen Alice Door and then Mad Tea Party. Opening it like a book, you could add the captions “You’re invited… Chez moi… To a party!” and the details of when and where. To deliver your invitations, you will need square envelopes, and remember that mail in special sizes will likely require additional postage. There, now, you’re done…go get ready for your party!