An old book or magazine or calendar (if you're intending to send these envelopes in the mail make sure the paper quality is robust)
Washi tape, decorated sellotape or similar
Double sided tape
Make the first fold. Make sure you have a minimum of 2cms of paper to seal the finished envelope properly. If the picture you have chosen doesn't allow for that, pick another one.
Check how your envelope is looking.
Make the next two folds. This creates the 'sides' of the envelope. I tend to do this to about 2cm but there are no rules here so if you want deeper ones, fold deeper ones.
Open your envelope out. The folds should be in roughly the same place as the picture below.
Now it's time for the cutting. Make your first two cuts as shown in the picture opposite. They will need some angle, cutting right across the fold line will make the envelope bulky when you come to seal it up. If you don't like to cut without reference, use a ruler and a pen to draw lines of the same angle on each side.
If you have a paper cutter, you can use it to make the second set of cuts on the largest flap, again cutting at an angle. Don't make this angle too wide as this part of the envelope needs to cover the sides without gaps. If you don't have a paper cutter just use scissors. This part of the envelope will be covered with washi tape at the end so if you aren't a very neat cutter it's not a problem.
When you've finished these two cuts, your envelope should look roughly like this.
Make the next set of cuts as shown on the picture below. Two on the side, two on the top. That's the paper cutting pretty much done.
Now, re-fold your envelope to check how it all lines up. Ideally when you overlap all the pieces (narrow sides ones down first, two larger envelope flaps down second) you should not be able to see the inside of the envelope poking out anywhere. If you have then I'm afraid it's back to the start with a new picture.
Now for the washi tape! Keeping the top of the envelope open, seal the sides of your envelope. I tend to put something on the centre of the envelope to weight the paper down to avoid bulging.
If this envelope is for your use and you are going to stuff the envelope then seal it with washi tape, your work is done!
However, if you have made the envelope for a silly gift for someone or intend to use it at a later date when you may not have any washi tape to hand, I would recommend placing a strip of double-sided tape along the envelope opening. This means it can easily be sealed when you or whoever you have given it to come to use it.
And that's it, folks! You can now go and unleash your creativity on all manner of charity shop bargains!