I make my husband's solid, but if you want adjustable, there is a pattern that uses one cord that runs around the whole apron in a casing that can be adjusted easily. I'll try to find the link.
Found it:
http://m.instructables.com/id/How-to...e-chefs-apron/
I uploaded this link from my tablet, so if it doesn't work for you, Google "how to sew an adjustable chef's apron"; it is on a site called Instructables.