In order to unlock this, you'll have to find 70 secret areas. Considering there are well over 200, this means you can miss quite a few. I highly recommend turning on level stats in the options menu as it shows how many secrets each level has and how many you've found. However, if you're a completionist, 3D Realms has an excellent walkthrough for the first four episodes that detail all the secrets contained within them. It is located at
http://legacy.3drealms.com/duke3d/walkthrough/. Along the way you'll also unlock the following trophy.
However, you can also get this trophy easier if you prefer by utilizing the rewind feature of the game. By grabbing a secret, killing yourself and rewinding to just before grabbing the secret, you can keep grabbing the same secret over and over until you unlock this trophy. For a video showing this technique, check out PurrPleeHaze's video below.