This year the first Sunday of August is also the first day of the month. To gain the indulgence one must go to confession during the eight days before or after August 1. The usual conditions apply: visit the Blessed Sacrament on the indulgence day (August 1), receive Holy Communion, pray for the pope, and be detached from habitual venial sin. There is no more toties quoties privilege, so one can only gain one plenary indulgence for oneself. I am not certain if that can apply to a soul in purgatory. Ask your confessor, or a knowledgeable priest about this.