Disclosure: This is not a method to 100% soundproof a room (we use the word "soundproof" because it's easy to use and understand), however, this is only going to reduce sound significantly to make a room quieter - not completely silent. It is great for a home office, kids rooms, etc.
One year later and I'm finally putting together a step-by-step on how we completed our project to soundproof our home office! I work from home 100% of the time and with 2 small kids in the next room, it is not always easy to conduct professional calls.
Since most walls are hollow, a good option, and the one we chose, is to remove the drywall and fill it with something to reduce sound. There are several ways to reduce noise but I'm going to walk you through this method.
STEP 1: Remove Drywall
You will need a razor blade to score the edges of the drywall (beside the molding). This will make it easier to remove the drywall.
STEP 2: Install Insulation
Once all pieces of drywall are removed, you can install the insulation. We used this insulation we got from Lowes. Place in-between studs and fill every space. Be mindful of wires and outlets.
STEP 3: Install Film
To add an extra layer, we added this sound reducing film we found on Amazon. It is super heavy, so 2 people are needed to install this. You will nail it to the studs.
Finish the project by installing drywall, as normal. Tutorial on that coming soon!
This method reduced the sound by about 80% in our home office. The office only shares one wall with the living area, so that's the wall we soundproofed. Good luck!