Follow these steps to create a new custom activity.
1. Click Admin.
2. In Database Management click Custom Activities.
3. Click New Custom Activity.
4. Enter a name and optional description, then click Next. The API Name auto-fills, but can be customized.
If you decide to change the API name, make sure the name does not conflict with fields in other custom activities.
5. Define your Filter and Trigger and click Next.
6. Give your primary field a name that summarizes what the custom activity is for.
And that's it!