I don't know if Buxtehude ever used any clefs. I thought all his organ music was in tablature, but maybe I'm mistaken. If it was in tablature, then the alto clef comes from an editor who transcribed it into regular staff notation. Clefs were used as ledger line savers.
Three ways to learn to read them:
1. enough exposure to make them second nature like treble and bass clef
2. thinking of them as transposing from a clef you know as already suggested
AND my favorite as a teacher.
3. read by intervals after you get the first note and don't worry about note names. This might improve your sight reading in general if you learn to read this way. In Wachet Auf for example, Once you determine the first note is B flat, you go fourth up, step up, Step up, repeat note, step down, third up, etc be mindful of the key you are in and think a little about accidentals.
It can also be helpful to learn Where all the C's and G's fall on the staff and then judge how far you are from one of those key notes.
By the way, G and F clefs are not fixed clefs either, G clef can be placed on the first line of the staff and it is then known as violin clef (examples exist all the time in Baroque music but most editions have "corrected" it to treble clef. F clef can also be found on the third line when it is then known as Baritone clef instead of Bass clef.
C clef can also be on the first line of the staff and it is then known as soprano clef. There as an edition of the Bach Chorales in soprano (c clef first line) alto (c clef third line) tenor (c clef fourth line) and Bass clef.
One of the tortures in solfege class at Juilliard was to require playing three of the parts and singing the fourth one and you weren't allowed to play any of the notes of the fourth part unless it happened to be doubled by another part.
John