Listen to the song and focus specifically on the drum part. Ignore the drum fills for now, but try and feel where the band places most of their accents. In this song, Iron Maiden place an accent on the last 8th note of every second bar. This is also known as the upbeat of beat four.
Count along with the song to better understand the crucial syncopation that the band uses so often. Accenting the upbeat of beat four (in a 4/4 time signature) creates a "pushing" feeling that excitedly moves the music along. McBrain plays his bass drum on beats one and three, and his snare drum on beats two and four. Count along first in quarter notes (one, two, three, four), and then double the counting speed and number of spoken words (one, and, two, and, three, and, four, and). Finally, feel the 16th note flow by again doubling the counting speed and number of words/sounds (one, e, and, a, two, e, and, a, three, e, and, a, four, e, and, a). It may become tiresome to vocalise these sounds for the entire duration of the song, so try alternating between speaking and hearing it in your head.
Play the main two-bar groove on the drums. If it doesn't feel comfortable at the song's original tempo, play along to a metronome instead. Set the metronome to roughly half the speed of the song, and gradually increase the tempo by five beats per minute until you are comfortably playing the groove at the original tempo.
Imitate the sound of McBrain's playing as well as the notes he plays. For example, during the main groove (and most of McBrain's playing, for that matter) he expresses time with the bell of the ride cymbal. He plays with the tip of the drumstick, which creates a less pronounced sound than playing the bell of the ride with the stick's shaft. Always aim for stylistic authenticity.
Listen again and focus on the drum fills. Most of them consist of 16th note single-stroke rolls on the snare drum and tom-toms, with the occasional flam (soft note directly followed by a loud note, played with both hands). If necessary, notate each fill and practice along at a slower tempo with a metronome.
Perform the entire song along with a CD or MP3 player. After a few attempts, the different sections should be well ingrained in your memory. If possible, record yourself playing with a microphone or video camera, as this is the best way to hear which aspects of your playing need improving.