Your piece of paper dimensions:
height - how tall you want the bag overall plus the depth of the base and the width of the fold if you are folding over the top edge
width - the total of the front, back and 2 sides plus about 1" overlap.
Adhesive - I like to use a glue stick but you can use any adhesive that will dry fairly quickly.
Wrap the paper around the form and glue it closed. It should extend past the form (the width of the base).
Now you can punch holes near the top of the bag if you wish, add brads for strength, and close with string or yarn. Or you can just leave as is, fill with tissue paper and add a gift.