Tonglen is the meditative practice of taking in the suffering of others and sending back love and peace. In order to develop true compassion we must be familiar with suffering. It means we explore what causes heartache or pain to others. This act is not the same as inviting pain into your life- it is the act of trying to feel as he or she does. Tonglen invites his/her pain away, transmutes it into love and tenderness, and sends it back in a warm bath of love.
Step 1- Breathe in suffering. The worst thing that ever happened to you. That thing you wish you could take back. Recapitulate it in breaths.
Step 2- Now breathe out joy, soothing golden warmth. See joy, feel joy, hear joy. Now do it for other people's suffering. Start with someone you care for. Visualize them sitting directly in front of you. Inhale his/her agony. Exhale comfort and transformation. Turn it into light and give back compassion and tenderness. Eventually expand out to acquaintances, people you've never met, remote communities and countries in need.
To learn more about the pain body and its role with the ego, I recommend reading, "The Power of Now" by Eckhart Tolle.