It is very thoughtful to thank individuals for their kindness following a loss. Sympathy thank you notes are very personal and what you say depends on how you feel about writing the notes. Some individuals create a template and use the same format for all notes; other individuals write more personal messages and use the experience as an outlet for their grief. Either way is fine.
You can say something simple, for example: "Thank you so much for the flowers in Charlie’s memory. It was a very thoughtful way to remember him. Your kindness is appreciated." You can add: "Your caring gesture was helpful during this difficult time."
If you want to write more personal notes to friends, neighbors, or colleagues you can certainly let them know your loved one mentioned them many times and their friendship meant so much to them. Or, if individuals shared stories in their notes, you can let them know how much their note and thoughts of your loved one meant to you. Whatever strategy you take is fine; the most important thing is that you’re acknowledging others' kindnesses. And you are doing it for your loved one.
Robbie Miller Kaplan is an author who writes from a unique perspective as a mother who has lost two children. She has written How to Say It When You Don't Know What to Say, a guide to help readers communicate effectively when those they care about experience loss, available in ebooks for "Illness & Death," "Suicide," "Miscarriage," "Death of a Child," "Death of a Stillborn or Newborn Baby," "Pet Loss," "Caregiver Responsibilities," "Divorce" and "Job Loss." All titles are in Amazon's Kindle Store.