During Kapalbhati, as and when you exhale, you should use your abdominal muscles (i.e. muscles around navel) to breathe out forcefully, and let the inhalation be passive (automatic) (no need to worry about inhalation - it will automatically occur), you just need to focus on exhalation using abdominal muscles in such a way the abdomen goes inside when you breathe out.
You are free to ask further if still you have any doubt.
