You will need:
2 1.5v cell batteries
a piece of plastic you can cut it out from a plastic soda bottle or a fruit container
Step 1: Building the Box and Circuit
First, draw this onto a piece of cardboard, cut it out, and fold it. You should now have a little box.
Now build the circuit.
Now you need a piece of clear plastic. YOu can get it from a plastic bottle a container of fruit or a CD case. Using a knife or a paperclip, scratch a reticle or a square into the plastic.
Step 2: Putting It Together
Next cut out 2 of the shapes in image 3 and glue them to the opposite sides of the box.
Then glue another clear peice of plastic on the front of the thing and a peice of cardboard on top of this.
Step 3: Paint It
To attach it to an airsoft gun either tape it on or hot glue or tape it to a removable rail.
Step 4: Red Dot Sight
To make a red dot sight, make a whole in the piece of plastic using an awl and glue an led into it. That is pretty much the only difference.
Step 5: Conclusion
I might add another step with some other easy to make gun accesseries to this instructable such as a verticle handle and some other stuff.
If you make one please post a picture.