Thank you for asking this question. It seems that this extraordinary provision for this jubilee year is quite under-reported by bishops and priests alike. In fact, I did not even know about it until I looked it up after reading your question.
You can read about it directly from the source:
In short, to obtain this second indulgence:
– obviously, all the usual conditions for obtaining an indulgence would need to be met, and some of them would have been met as you would have already obtained your first indulgence: e.g., in a state of grace, detachment from all venial sins, gone to confession or will go to confession within 20 days prior or after, unless of course, alas, you have already committed a mortal sin or reattached yourself to venial sins since the first indulgence was obtained.
– the second indulgence can only be applied for a soul in purgatory, and is obtained through performing an act of charity for this soul.
– it can be obtained only within the context of holy mass; namely, one must participate fully at a second mass, and ask for it during that mass, in order to obtain it for a holy soul; this of course would automatically satisfy the remaining conditions for obtaining an indulgence, namely, receiving Communion and praying for the Pope’s intentions.