The US National Park Service which manages San Felipe del Morro offers guided tours of the grounds on Saturdays and Sundays at 10:30 a.m. (English) and 2:30 p.m. (Spanish). Unfortunately, the US National Park Service provides tours of San Cristobal at the same time as its tours of San Felipe del Morro. So, if you want to take the English tour of each fort, you will have to go to one fort on one day and the other fort on another day.
The US National Park Service also provides a multi-room exhibit on the history of the fort and a 15-minute orientation talk every hour in the main plaza. While you can learn about the fort without going on the tour or listening to the orientation, the tour does provide a perk of taking visitors to the top of Castillo San Felipe del Morro Lighthouse which is not otherwise accessible to the public.
Once inside, you will climb this spiral staircase and then another staircase. For ladies, do not wear skirts as the winds are strong and will blow every which way imaginable. The Park Ranger was nice enough to suggest that ladies with skirts would go last to preserve their modesty. 😛 Also, do not wear flip flops or sandals as the steps are more like ladder rungs. Once, you have reached the top and stepped out onto the ledge, you will be met by cool winds and fantastic views.