There are many ways to write a note, I think it should be personal, as stated by nightlygem above little bits daily is the way to go... I normally find examples help me, in this case... Here is mine, to my friend, I hope to one day never need this.
"Hello [Friend]
If you're reading this, then I am dead, I have committed suicide. I have walked into the sun...
Life was hard and painful, bar you there was no good in it... I had enough the loss of my father affected me more than I ever dared say, the loss of him at an 5-years old caused my life to spiral into the gutter in a trajectory that could not be fixed. My own mother didn't love me, my own brother saw fit to abuse me as child... The damage from these two were eternal, and I could not fix myself. I was trapped in a damaged body and a damaged soul.
Couple this with the childhood bullying, the poverty, the obesity from said poverty, the autism... My self-hatred of my mixed-race nature, and my awareness of people's value based on their outwards genetics. I was finished before I even began and could not see a way out, I was destroyed as a child, as a 13-year old when I got my first depression diagnosis, it just took till 27 to finalize this. I lived a long life, and it was all pain and misery.
You, and [Boyriend] both tried to set me on the straight-and-narrow but this proved to be futile, there was too much damage and too much pain to fix me, I was already damaged irreparably by the end of the [Secondary School]
You were my best friend during those tumultuous years, and one of the few bright things I could look forward to in this life. My spark of hope in an otherwise endless darkness of bullying and child abuse. Even after the culmination of the [Secondary School], you were there, offering a shoulder to cry on, advice, and more than that... Friendship.
You, [Friend], were my best friend, and I am eternally grateful I ever got to spend time with you. I was lucky that, I ever got to call someone like you friend.
I ask that you do not let despair take you, as it took me, fight it with all of you might and beat it. This marks my end, my final! But for you this tale goes on, I ask that you do what I could never and live a meaningful life with your family.
I will see you on the other side,
Goodbye [Friend] my best-friend in this life.
Life was the night and I walked into the sun."
I hope this example from a fellow SaSu user helps you, no matter what you choose. This note was/is to my best friend,