My advice is specifically for those who want to forgive. In other words, they are ready to do so. First, I would recommend doing a little research on those who have been identified as planning and perpetrating the attacks. I say that so that you can concretely see who the people are. It is easier to forgive a person you can see rather than an abstract set of people.
From there, I recommend working on seeing them as persons. You can gain some insight into this in??the first Adult Forum on this website.
Next, you might want to consider what we call “bearing the pain” of what happened. You are not giving in or taking on pain that will not leave. When you bear the pain you commit to not passing the pain that you now have onto other people. Sometimes we displace our pain onto innocent others. As we bear that pain, the paradox is that it leaves.
Finally,??you might draw inspiration from this video.??It concerns one mother who lost her son in the attacks on 9/11 and another mother whose son was convicted in taking part in those attacks. They have formed a bond through forgiveness.