Seem to be behind with everything still but the
Arty Girlz challenge ending today (!) was 'Alter It!' and I had in mind to use these US postal envelopes to make an expanding file: Got to find some way to get organised!!!

So set off by cutting them in half, scoring in at about 2cm each side and cutting the bottom corners off at an angle so I could fold them in for the 'expanding' part:

Stuck 5 of these half-envelopes together and then set about covering them.

I used 2 pieces of docrafts 12"x12" paper and measured the front with a flap + gusset underneath, concertina sides and a flap over the top edge:

I did the back similarly but without the underneath part, some extra concertina bits to finish the sides, then some extra pieces over the tops of the inner bits to neaten where the envelopes join.
Quite pleased with how it turned out:
