First thing you must do before you construct any kind of room is lay out all of your materials. This will makes things go a lot faster and smoother.
Once you have everything laid out, pick the room that you're going to soundproof. In order to find out where you need to put insulation you're going to have to find the "sensitive" spots of the room. These spots are easy to find if you do the "Hand Clap" method. This involves standing in the middle of the room and clapping your hands real loud. Wherever the sound bounces off the walls at, that's where you're going to need to put insulation. Go around the whole room until you pinpoint all of these spots.
Start by putting a layer of carpet down on the wall and securing it down with nails. Crazy glue may aid in helping the carpet stay up on the wall a little better. Note that some people will simply insulate a whole room if they want it completely "dead". However; padding down the whole room will also cause your vocals to sound "emotionless", meaning they have no natural reverb from the room.
Once you've put the carpet on the wall everywhere that you want it, start placing the couch foam over the carpet layer. You will need to secure the couch foam down with small nails since it's thicker than the carpet. Try not to overlap the couch foam, as it will affect your overall vocal sound. The vocals will bounce off the uneven foam sometimes causing your vocals to sound slightly distorted.
Once done with padding, do another sound test. This time do the "Yell Test" meaning stand in the center of the room and yell to see if you hear any echo. Having a little echo is a good thing, however; if you don't have enough padding you will hear the echo bounce off the wall which is a bad thing. That just means add more padding around those super sensitive parts of the room. Once you have the room how you want it, leave it! You can now begin your recording in your soundproof room!