One sheet appropriately sized paper. Fold round to make 'bag' shape. I've folded the top edge over to neaten, also strengthens if you want to add handles at the end.
Glue edges. Fold section up from base – bear in mind that the final whole width of the flat base to your bag will be half the amount you fold up now. Fold back over the other way as well.Open out and fold 'flaps' thus formed into centre and stick down.
Fold flaps in half again and stick down again – tada!If you wanted you can pop a strip of tape on the bottom for extra safety. Then punch holes in top – add ribbon handles / tags &c &c.