Visit the Cathedral or Co-Cathedral Church of your Diocese, or a Franciscan Church, or your parish Church (quasi-parish churches qualify) between noon on August 1 and midnight August 3. While there, recite aloud an Our Father and the Creed.
There are two general requirements for gaining a plenary indulgence:
Performance of a designated good work or act of piety;
Freedom from all attachment to sin, even venial sin.
Besides these, there are three conditions which must be fulfilled for any and all plenary indulgences (except the one granted for the moment of death):
Sacramental confession within 8 days before or after. However a single confession suffices for several plenary indulgences.
The reception of Holy Communion, once for each indulgence.
Prayer for the intentions of the Supreme Pontiff. One Our Father and Hail Mary are sufficient, but other prayers for the Pope's intentions may be said. This is required for every plenary indulgence, except the one for the moment of death.