cut a piece of the t-shirt fabric to the size you want. measure this by folding it over into a pouch-shape, to see how big you need it.
Once you have a rectangle that is the right size, pin it to the nice fabric (black flowery in my case) and cut directly around it.
Get your sewing machine out, and sew round one short side, and two long sides.
for the other short side, sew about an inch (depending on the size of your pouch) from each edge towards the centre, leaving an opening big enough to turn the whole thing right way round. This is to made all the corners nice and rounded.
turn the pouch right way round, and sew up the opening which is left. You should now have a long rectangle with black fabric on one side, and pretty fabric on the other.
fold it over into a pouch shape, and hand-sew up the sides of the pouch. I found that it helped to sew through just the two layers of black fabric, because i don't think i could have managed sewing through four layers of fabric!
Finally, hot glue the velcro strip to your pouch, and you are done!