What you need.
Measure the width of the wine bag (10cm in this example)
Allow space at top for closing the finished bag + the height of the gift wrapping/ box + half of the bag width (10/2 = 5cm in this example). Mark position with pen.
Cut the off bottom off bag at this position.
Fold up the cut edge of the bag at the half width mark (5cm in this example) then fold back to crease the paper well.
Open up the bag and fold those sides into the centre at the creases. (my scissors are there just to hold the folded sides in place).
The hard bit: Lay the bag on it's side and fold the sides in, at 45 degrees. Half of one side is shown folded.
Tape across the join.
Repeat on other the side
Fold and compress the bag until it folds flat, Sellotape across bottom (Sellotape across the bottom on the inside as well if you feel it necessary.)
Open up the bag, insert gift and sign the card.
Finished gift bag. Done! The top edges of the cut off base can be folded to the inside to make a second small bag where the opening can be stapled, taped or ribboned shut.