First I flipped the frame over so I could access the back. Then I pushed the thumbtacks evenly into the frame over the opening. (Sorry for the blurry pic!)
Next, I measured the fishing line in different lengths to loop through the handles of the watering cans, and then tied them around the thumbtacks. I didn't measure anything, I just eyeballed the whole project.
After I had the fishing line tied to all the thumbtacks, I hung the frame back on the wall. Then I used wire cutters to cut stems of the flower bunch I picked up at Hobby Lobby, then stuck the flowers into the watering cans. You could use flower styrofoam to make the flowers stay just so but I didn't do that.