Creating a curled banner is easy, but it creates several open paths. The Join Tool works great for joining shapes but trying to create complex closed shapes is not as easy. Here is a quick trick with the Shape Builder Tool can create closed shapes from complex open paths.
Start with a circle, no fill, heavy stroke.
Duplicate the circle so the edges touch.
Move the one circle down while keeping it in line.
Select the bottom of the top circle with the Direct Selection Tool and delete it. Do the same with the top half of the bottom circle.
With the Direct Selection Tool select the inner open ends of the circles.
Join the ends together by choosing Object>Path>Join.
Duplicate the lines.
Join the ends together by choosing Object>Path>Join. Do this twice to join both open ends.
Use the Direct Selection tool to remove the 1/4 circle from the top and the 1/4 circle and line from the bottom.
This is what it will look like.
Draw lines to close the shape in the 3 indicated areas.
Use the Direct Selection tool and select each intersection and join them together. Do each one of the three steps.
Select the shape and make it narrower to create a banner
Rotate the banner. Add circles filled with the same green color as the stroke, add a white stroke around the circles, place them along the line.
If you move the parts of the banner, you will see that they are not closed paths. Even the top section looks like it is closed (on the left side) but it is not! The open ends are hiding behind the lines (shown by the red circles).
If you try to fill the areas with color, the paths are not closed. Filling open shapes creates weird tangents.
What you want to create, are 3 closed shapes. Each one being independent of the other shapes.
To make closed shapes out of each object, select the banner, then choose the Shape Builder Tool (SHIFT + M) from the Tool Bar. Click on each shape you want to create (the mesh will appear on each shape you click on),
Each section you click on with the Shape Builder Tool will make it into a closed shape. Now each object is a closed shape. (I took it apart to show each shape).