This short writing practice can be really cathartic and is a great way to put positive, loving energy into an otherwise difficult situation. You don't need to be ready to forgive anything in order to practice what it might feel like to do so.
If there is a person in your life that you feel has wronged you in some way, then thinking about them or what they did probably carries a charge. This person might even be you! Maybe when you think about them or the situation you feel a tightening somewhere in your body, or you think negative thoughts. Practising forgiveness, even if you tell yourself that you absolutely don't mean, may well make you feel a little better. This exercise isn't about them, it's about giving yourself a bit of loving care.
To try it, grab a piece of scrap paper. Tell yourself that you are going to tear it to tiny pieces after you finish writing - nobody will ever know what it said. You don't even have to read it back to yourself if you don't want to. You may choose to write the person's name at the top of the page or not. Then write them a letter of non-forgiveness! There are many different ways to begin but my favourites are:
'I don't forgive you but if I did I'd say....'
'I forgive myself for not forgiving you for....'
Those openings give you total permission to write whatever you like, don't they? Nobody's going to read what you write. You don't even mean it, so go to town! Practice the language of forgiveness and see how it feels. If you can, write quickly. Keep your hand moving, without giving your head time to think too deeply about your words. It can be messy. It can even be nonsensical in parts. Who cares. Just let whatever wants to come out, out onto the paper.
Observe how you feel afterwards. Has anything happened to that charge? How intense is the tight feeling in your body? How do you feel about the situation, the person, yourself and the subject of forgiveness now? You might be surprised. Words really are magic you know?
Just a little bit of what's going on.