(note: apologies if this is in the wrong forum, hopefully it'll get moved to the correct one if it is? )
Hopefully, I wasn't the only one who saw the envelope and had to work out how it was done!
Well, anyway, I spent this morning reverse engineering the little thing 
(yes, I have too much spare time)
Unfortunately I'm not an activated giffgaff-er yet, I'm waiting till my o2 month expires, after which I'll be ready to go! ;}
we're going to make this today!
So, without further ado, here's how:
1 - get your paper and lay it out on a hard, flat surface. It doesn't matter what paper size you use, so long as it is one of the standard sizes i.e A3,A4,A5 ... Etc. We'll be using A4 , because that's what the giffgaff envelope uses.
2 - lightly fold or otherwise mark the horizontal centre line so you know where it is ... I tend to very lightly fold the very top & bottom of the line, so as not to interfere with the folds of the envelope itself.
3 - take the bottom right corner of the paper, and fold it over so the inner edge of the corner now sits on the horizontal line:
3a - and repeat the same for the opposite side, taking care to ensure that the edges meet in the middle, but do not overlap:
4 - now take the top edge of the paper and fold over, so it meets the edge of the fold from 3. :
4a - and, of course, repeat the same on the opposite side...
5 - next, take the corner from this new shape, and fold it back on itself, making sure it does not overlap the other half, or protrude from the shape.
5a - repeat the same on the other side
6. Put a sticker over the centre of the envelope, to hold things together .
And... You're done! One envelope, ready to go
send some love to giffgaffers and non-giffgaffers alike!
@rhinosrich - sort of missing the point - this isn't for handing out of sims, just for making the envelope for... well... letters? what is an envelope used for? 
Also, i made a design template for this - i'm nearly 100% sure this will work perfectly if you have an accurate printer (unlike mine) so you can add designs and colours and all manner of interesting things to your envelope, or use the inside of the envelope as the main body of the letter, like giffgaff did :}
(click the image for full size)
a quick improvement here -
when you've finished the enevelope, even after sticking the back, you might find that the two flaps ... well... flap around a bit.. as my friend pointed out, you this might get trapped in some kind of automated machine over at royal mail or similar ( though i dont think giffgaff's ever had a problem?)
well anyway, the solution to this is -
if you lift up one of the flaps, you'll see a triangle shaped area created by the opposite flap. you can pop the end of the flap under this, and it'll be held down by the opposite flap.
repeat this for both sides, and you'll have something that looks like this:
now both sides are held in at the corners! great!
obviously you should still put a sticker over the middle, as this will make it nice and secure, because anyone could open and re-seal your envelope if you left it as it is here.
