Step 1:  Figure out what color your costume will be. 
Step 2:  Get some material of that color, such as fabric, plastic, paper or wood. 
Step 3:  Get some glue, some paint in an alternate color you like, and some structural materials such as wire, bamboo, thin wood, pantyhose, Styrofoam, etc. 
Step 4:  Try to put it all together in some coherent way. 
Step 5:  Become frustrated about the pile of crap you have concocted.   
Step 6:  Take everything to Freon’s basement and beg his help.  Cry, whimper, and offer gifts of food and backrubs*. 
Step 7:  Watch him turn your stuff into costume magic. 
Step 8:  Get rave review at your next Con. 

 *The views and opinions expressed in “How To Costume” are not necessarily those of the Great Piñata Riders Propaganda Rag.   The backrubs, of course, are appreciated, but there is no guarantee expressed or implied that this procedure will in fact yield a prize-winning costume.  It will, however, make you a really good friend of mine.~freon