If I were to try to explain how to achieve compersion it would be to learn to care about your partner's partners. That is where my compersion comes from.
I care about Polynerdist enough to step aside without hesitation if they wanted me to. I care about Derby and I care about RP's tertiary.
That is one of the reasons I believe in order to have a sustainable integrated muti-partner relationship you need to not only know each partner, but also need to possess genuine caring for them. The extension of your love for your partner through them and into their other partners. That is my key to compersion. But my compersion isn't your compersion...compersion is as fluid as poly itself