I can relate to what you are going through with the thought of 'messing up' but you won't! It will be a beautiful memory when you are finished. It is awesome that you are doing this for someone...make sure you take a pic of your accomplished quilt & you will have a memory as well!
I also made a few memory quilts with clothes for family members; but didn't take pics. It's a hard thing to start with the first couple shirts, but it will come for you!
Bless you for doing this for the family