I love creating custom envelopes because you can do it for cheap as well as it is simple and fun! I found this idea on Pinterest the other day and was able to try it out this morning. This is how I did it and it worked really well.
I took a 4 x 6 inch invitation envelope.
And carefully opened it so I didn't tear the main part of the envelope.
Then I took a piece of construction paper and made an outline. (I recommend using paper that can be recycled for practice purposes so you don't waste your good paper if you make a mistake.) I also find it helpful to make where the folding lines are and where the top of the envelope is.
Here is what it looks like when completed. I usually use clear tape to seal mine (I used double sided sticky tape for this demonstration), but depending on the thickness of the paper you will be using for your envelopes, I believe you could also use a light amount of glue.
I also recommend tracing your completed envelope pattern onto cardboard so it can be repeatedly used.