Many pens are assembled from scratch and some are modified LED-lamps where you only replace the LED with an IR-LED
It all depends on how much you are going to use it, how you are going to use it and your basic skills regarding electronics (soldering and such)
To aid calculate resistors and such there are a number of applications available on the web including this one that I really like:
http://ledcalculator.net//UndCon
