This one is super easy and can be gotten on any level in any Doom (aside from a few that don't have them). It's easiest to get it in Ultimate Doom's E1M1. Turn around when you start the level to see a small room with a staircase. On the right pillar before you go up it is a button. Hit it and quickly turn around to find a door has opened on the wall behind you. Rush through and grab the Rocket Launcher then down the staircase to the right of the pit. Grab the goodies and hit X on the wall at the end to open the secret.
You can get this an alternate way as well. From the start, head along the hallway and go through the rooms until you get to the room with the large acid pit that has small walkways you have to walk across. Get to the other side of the room and into the small hallway. Before you go through the door, look at the wall on the right. Notice how a section of it is slightly different? Press X on it and you'll open the wall. Go in and grab the goodies to unlock the secret.
I've posted a Youtube video, which shows you how to 100% the first episode. This is NOT mine and all credit goes to BigMacDavis for this one. While most of it is unnecessary, it will show you the level's secrets as well if you're having trouble.