Well I have tried this method I came up with on just about every size card I can think of and so far it works great. I hope you will find it works for you too. I see many videos on YouTube about how to come up with the sizes paper, folds, score lines etc to make any size envelope but they all see to have so many steps in them. When I make a card and need an envelope I don't have a lot of time to I need something quick and easy. After all in this fast paced world isn't that the name of the game fast?
Full tutorial Below
Or: https://docs.google.com/document/d/1-HzQru-HRQv1162Z-JzcSw8SutZLQ5P7wUcjpYHhWZ0/edit?usp=sharing
So I came up with these steps and have tried them over and over and they work every time, so far! And it is only two basic steps. I have worked out the math for most normal card sizes and have created templates, linked below, for all that I can think of. But if you have a different size card just use the steps below to determine what size paper you will need for your envelope. And these can easily be made with pattern paper, I just used card stock to make it easier for you to see.
The first step is to determine the size of your finished card. Now add 1/5" onto both those measurements. This will be the size of your template, and yes make you a template and write the size on them, makes it much easier when you want to make another envelope for that same one.
The second step after the template is to determine what size paper you will need to make your envelope. So if your card is anything other than a square card like a 4 1/2x6 1/4 or a 4 1/4x5 1/2 then you will need to take the largest size which in the 4 1/2x6 1/4 is the 6 1/4 and add 3 inches onto that. Thus your paper size for the envelope should be 6 1/4x3 = 9 1/4x9 1/4 make the paper square.
Then just take your template, lay it as shown below, with the center points of the top and bottom of your card facing the points and fold around your template. As you fold around fold the opposite sides, this way you can match your points to make sure you get it close to even all the way around. I explain this in the video.
Take the template out and reinforce your score lines and then trim out where the score lines cross over in the corners. This makes the card fold over must easier. Glue and you are all set.