For the average person, it doesn't really matter how a letter, check or card is put into an envelope. Their recipient will simply open the envelope, pull out the contents and throw out the envelope. However, when it comes to filling envelopes for business purposes, there is absolutely a proper way to stuff an envelope. That's because it's important to create the best first impression possible when your recipient opens the envelope.
When you send a business letter to a recipient, the first thing you should do is fold the letter into thirds, starting with the bottom third of the letter. Fold this backward so that the bottom third of the first page is now sitting on the back of the last page. Next, fold the top third of the front page and bend it back in the same way. When you look at the folded letter, you should only see the top third of the first page on one side and the middle third of it on the other.
Before stuffing the envelope, determine whether you will be using a window envelope or a standard envelope. When using a window envelope, be sure that the part of the document with the recipient's address is inserted in the envelope so that it is visible through the window.
Most business documents have the address listed at the top of the first page, so you should just put the top of the letter facing the front of the envelope. If you cannot get the entire address including the full ZIP code to fit properly inside the window, remove it from the envelope and use a windowless envelope with the address written or printed on it.
If you are using a traditional envelope without a window, then the top of the first page should be visible when lifting the flap so the recipient can start to see what was sent before she even pulls the letter from the envelope. This is the best way to help ensure your document will be read and also makes it easier for your recipient to start reading as soon as she removes the letter from the envelope.
The rules for checks are mostly the same as they are for letters, only you want to guarantee the check will either appear in the window of a window envelope or be the first thing the recipient sees when opening a standard envelope.
Since most checks printed on full-sized pieces of paper have the check on the bottom third of the paper, this means folding down the top third first and then the bottom third so the check is visible on the top of the folded paper.
While it is preferential to put a business letter in a standard envelope so that the first page faces the recipient, that's just a way to increase the chance that your letter is read. When it comes to putting greeting cards into an envelope, though, there is a well-established set of rules defining proper greeting card etiquette, and these rules state you should ensure the front of the card is face up toward the flap. This ensures your card will be immediately visible when the flap is lifted and that the recipient won't need to flip the card over to read it. This is also the proper way to insert an invitation into an envelope.
Proper etiquette also dictates that you should put the card seam at the top of the envelope by the flap. This is important because it ensures that any money or gift cards put in the greeting card will not fall out as soon as the card is opened. You should always put money or gift cards inside the greeting card rather than leaving them loose in the envelope to ensure they are seen and do not fall out when the card is pulled from the envelope. This is particularly important in helping to ensure these gifts are not lost.
As for writing a birthday envelope message or adding stickers to the envelope, this can be done on the front or back of the envelope, but if you are mailing the envelope rather than just handing it to the recipient, make sure the mailing address and return address can still be clearly and easily read by postal workers.