Now that you're inside San Jerónimo el Real, you can recognize the few late-Gothic parts of the church that have survived its repeated restorations: mainly they are the ceiling with a cross-shaped vault and original Byzantine-style chandeliers, and behind you the choir, or the space reserved for singers which is occupied by an impressive organ.
As you can see, the church has only one nave that's flanked by five chapels on each side, where you can admire some remarkable altars and noteworthy paintings, including a beautiful penitent St. Jerome in the fourth chapel on the right. The chapels are automatically lit.
I recommend taking a few minutes to admire The Last Communion of St. Jerome at the back of the church. You should know that this nineteenth-century painting is an impressive 8 by 4 and a half meters, and is one of the largest paintings in Madrid and probably all of Spain: just think, it took twelve strong men to place it above the altar!
At the end of the restoration completed in 2011, two images of intense devotion were placed on the right and left of the main altar: these are the "Christ of Good Death" by Pedro de Mena, and on the opposite side, the "Lady of Sorrows" by Jerónimo Suñol. They are among the few works that can still be admired within San Jerónimo since the Archdiocese sold most of its artistic heritage to the adjacent Prado Museum.
Next to the last painting you'll also see the church's rear exit, which opens up onto Calle Moreto.
FUN FACT: many official ceremonies were held here before the Almudena Cathedral was completed, including the solemn Mass of the Holy Spirit for the coronation of Juan Carlos I after Franco's death, and the baptism of the current King Philip VI.