Summarize:

Discard the cut out portion.  This can be done using a can opener or the tin snips. Experiment with what works best. A halo can be made by gluing on a golden or silver colored pipe cleaner shaped into a halo. One can will form the body and head, the other will be cut to make wing shapes: This will be the body of the angel and the open end is slipped onto the top of the tree. Cut out the design to use as a template. See Warnings below about sharpness. Flatten the can. Cut around the wings. Discard the unused portion.  This becomes the "head" of the angel. Stick the angel at the top of the tree.
Wash and let dry the beer can. Draw the shape of a dress, wings and head onto the can. With great care, cut away all of the unmarked portion of the can, leaving the dress, wings and head design attached to the top and base of the can. Gently push the wings forward so that they are not still curving backward. Cut out the base of the beer can to allow it to slip over the tree. It's up to you if you'd like to completely remove the top of the can or leave it as part of the final ornament. Hang on top of the Christmas tree.If you don't want to cut out the shape in this way, another alternative is to use two beer cans. Cut off the base of one of the cans. Draw two wing shapes on paper or card. Cut open one of the cans using tin snips. Attach the templates with poster tack or similar. Glue the wings to the back of the can, at the top portion. Tilt the tab at the top upright. Attach a golden pipe cleaner halo to the tab. Hang on top of the Christmas tree.