Ehow has some great "how to do it" articles. Unfortunately, they do not come with pictures. Wondered what it would be like if they did. So here is the original ehow article with pictures and a few of my own notes.

From http://www.ehow.com/how_6456375_make-antenna-out-tin-foil.html

Notice: Your results may vary!!

Step 1: What's Needed.

Aluminum or tin foil
Glue stick
Construction paper
Scissors
Tape

Step 2: Step 1.

Unroll a square of aluminum foil sized 1 foot by 1 foot. Keep the foil as smooth as possible so that it will be a better reflector for the signal once your antenna is constructed.

Note: Do not have a 1 x 1 foot section, but it could be cut out.

Step 3: Step 2.

Cover the dull side of the foil with glue from the glue stick. Spread glue evenly out to the edges, and be sure not to tear or bend the foil as you go.

Note: I just used regular glue and spread it out.

Step 4: Step 3.

Glue the foil square to a piece of construction paper or other thick paper. Spread the foil out evenly to make sure it sticks well to the paper.

Note: Poster board could work. (Let the glue dry).

Step 5: Step 4.

Cut the foil and paper into a long rectangle shape, approximately 6 inches wide by 9 inches high.

Note: You should be able to get two sections out of this. Most routers have two antennas.

Step 6: Step 5.

Bend the foil and paper into a concave parabolic shape with the shiny side on the inside. This will serve as the reflecting dish for your signal.

Note: you may have to encourage the pieces to go in a parabola shape. We left them there for a while.

Step 7: Step 6.

Cut two holes in the parabola about the width of a pencil and located halfway up and 2 inches from either edge on the sides.

Note: The directions, just did not seem right. The upper antenna is what I thought they wanted. The lower antenna made more sense.I Just made a cross-cut with a knife the width of a pencil. If it were me I would have done this step before the last one.

Step 8: Step 7.

Add structural stability to your parabola by creating a support beam for it. Roll a strip of construction paper into a tube slightly smaller than the width of a pencil and stick it through the two holes in the parabolic dish. Your parabola should now resemble a "D" shape with the roll of paper representing the long straight piece and the parabolic dish curving around. Use tape to secure the ends of the tube to the construction paper side of the dish.

Note: I did it both ways. You can choose what is best for you.

Step 9: Step 8.

Place the dish over the WiFi antenna on your existing router and direct it toward the computer you wish to send the signal to. Secure the dish by attaching the paper support beam to the existing antenna with tape. Your dish will yield up to 9 decibels of signal boost.

Note: I just attached the reflector directly to the antenna aerial. Router runs a redboot install version of dd-wrt instead of the factory firmware. D-link made most of the routers that sold either under the Airlink or the Frys name.

Step 10: Easier Way Maybe.

Take a cheap foil platter that you can get two for a doolar.
Mark two 6 x 9 inch sections.
cut them out. Mark for the holds as you did in an earlier step.
Punch the two holes in each unit.
Wrap the edges of the foil so they are not sharp such as taping the edges over.