The first step in making a memory quilt is simply to offer to do so. Understand that the bereaved may have very different reactions to your offer, however kindly meant. They may not want to keep the old clothing around or to even have the daily reminder presented by such a quilt, at least for a while. They may not be able to let go of the clothing for some time. Simply offer and let them get back to you when they are ready.
How many items of clothing will you need? It will depend on the size of the quilt and the pattern you choose. I suggest you make these choices based on what clothing times you receive, instead of the the way around. Let the bereaved give you what they want, rather than dictating what you need. See what you can make with what they offer. Maybe it's only enough for a pillow or a set of pillows for a sibling group. That's fine. Maybe it will be enough for a throw or a twin quilt. Since it is primarily for comfort, I chose to make a throw quilt.
When you have chosen the clothing items you will use, you will need to deconstruct each item. This can be a sad process, but I promise it gets easier as you start thinking about the snuggley quilt to come. After contemplating using a seam ripper to take apart a large stack of dress shirts, I quickly realized I would be better off cutting the seams out. It's much quicker, and the fabric in the seams is often less faded that the rest of the shirt or hopelessly marred by thread holes. I cut off collars and cuffs and discarded them, but you may wish to preserve pockets if you want to feature them in your design. Once the clothing has been cut into usable pieces of fabric, press it well. Using starch makes cutting and piecing even more precise later.