Your letter is received. You wish to know how to send peace and love to souls who left their body. In other words, you wish to ask for forgiveness. So here is your guide
Forgiveness means Acceptance + Understanding. So they accept your mistake and you give them the light of peace and harmony. This way, your karmic account will not develop ahead and both you and that soul shall feel light.
* Steps *
1. Wake up early morning and sit first in silence. Try to focus you mind on single thought. ''I am a soul.. I am a Soul..''
2. Practice this swamaan: http://bit.ly/Swamaan (Video commentaries)
3. Then imagine the persons in front of you (whom you ask for forgiveness - your grandparents, etc..)
4. Imagine the soul shining in the centre of their forehead. Then send them the energy of love and peace. And with your heart, you truely say them 'sorry'.
Is that clear?
Practice Swamaan daily morning. This is enhance your concentration power and Soul consciousness.
What is Raja Yoga meditation?: https://www.brahma-kumaris.com/rajyoga-meditation
Here are some videos and a forum post that shall guide you.
How to forgive the self? - BK Shivani: https://www.youtube.com/watch?v=neMTGM_S7eA
How they can forgive me? - BK Shivani: https://www.youtube.com/watch?v=ZlGXQzT9ie0
How does forgiveness work? - BK Shivani: https://www.youtube.com/watch?v=6DhrOyAkM3w