I don't have a step by step guide to offer. Forgiveness is such a personal thing. These are just some suggestions that have worked for me. I want to stress that because you are forgiving yourself doesn't mean you did anything wrong. It is simply a way to relieve burdens in your heart.

Place one hand on your heart and say one (or all) of the following. You can repeat a statement over and over til you feel relief, or say it once and just feel. Notice what you feel without trying to force a feeling.

Customize statements that work best for you:

"I forgive myself for not being able to forgive".
"I forgive myself for carrying this anger".
"I forgive myself for being unable to sleep".
"I forgive myself for the flashbacks I am having".

Another suggestion is any time you are going thru mental anguish (or when you feel like a blessing) place a hand on your heart and make this simple statement:
"May I be blessed".

You could be surprised at the peace and relief you feel.
I wish you the best of luck and peace in your life.