1) Notate all cadences. A cadence is a melodic or harmonic figure which indicates the closing of a musical idea. Note if a cadence ends on the tonic chord (final cadence) or on the chord other than the tonic (half cadence).
2) Find all important modulations. A modulation is a key change with a cadence. Usually the modulations in such a piece will be to closely related keys. In this piece such keys either have the same number of accidentals or differ from the home key by only 1 flat.
3) Find all sequences. A sequence is a harmonic or melodic idea which is repeated in an ascending or descending order. Sequences are perfect tools for travelling from one key to another. In such case, the new accidentals usually appear gradually one by one.
4) Count the number of fugue subjects. This fugue opens with a theme or a subject which is 6 measures long. Go over the entire fugue and mark the numbers of the themes with pencil.
5) Indicate which voice has a subject. Take a pencil and write in S (soprano), A (alto), T (tenor) or B (bass) next to each entrance.
6) Notate tonal areas of each subject entrance. Remember to check for additional accidentals which will be determining factors for different key areas. Similarly to the prelude, in the fugue the modulations will be to closely related keys.
7) Mark scale degree numbers of each subject entrance. Keep in mind the home key of B flat major. Then write the scale degree numbers or chords (T, S, D etc) relative to the tonic key. For example, the subject in B flat major will have 1st scale degree or the Tonic while the answer in the F Major will have 5th scale degree or the Dominant.
Apply the above 7 steps in analyzing the form of this piece today. While doing so, you will put yourself in the composer's shoes and will have a much deeper understanding of how the piece is put together and be ready to start practicing it.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my FREE Organ Practice Guide.
Or if you really want to develop unbeatable sight-reading skills, check out my systematic master course in Organ Sight-Reading.