Sewing chalk (Or something to mark fabric with)
Thick fabric of your choice
A small amount of Velcro (sticky or non sticky depending on preference)
(If you don't want to use Velcro, you can use snaps, buttons, or leave as-is)
(The top flap is folded in this picture, but you should originally cut it out as a rectangle)
Hem the edges marked in red on the first picture. (Hem the flap as a rectangle before folding)
If you would like your bag to have lining, now is the time to cut the lining in the same shape, hem it, and attach it to the bad side of the bag piece.
(See Yellow marks on first picture)
Find the center and fold the sides so it forms a triangle shape, then fold the tip of the triangle down just like wrapping a fancy Christmas present.
(If you are going to sew on your Velcro rather than use sticky Velcro, now is the time to sew it on.)
Iron again so that the sides fold inward as shown in pink in the next image.
Basically, you've got a bag that's way bigger than it's cover flap at this point. You want to fold the sides in so it's all an equal size. These side folds look the same as the sides of paper bags.
As for the bottom corners, push those inward and down. They will fold in with the sides and not stick out.
PS. I hate sticky Velcro. It pulls off sometimes, it ruins your sewing machine... I usually do not use it but for this project it worked. Also I was crunched for time.