Take an A4 sheet of paper and fold it in half lengthwise.
Open it and fold this part towards the middle line.
Do the same thing with the opposite corner.
Then fold the edge upwards, like this.
Do the same on the other side.
Once you have this shape, fold this part down to make a flap.
Repeat for the other side.
Then slide your flap under here.
Do the same thing for the other flap.
And your envelope is done.