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Sequences move the harmony around by a preset amounts, usually a pair intervals of opposite sign: plus five, minus four. This means the bass will leap up by a fifth, drop by a fourth, then repeat. There are not too many sequences given the limited intervals usually involved (2, 3, 4, 5) and not all sequences may be usable. Variation is possible by using 7ths instead of triads, or substituting some other chord that is "close enough". Some of the sequences do not "go anywhere" harmonically, which means you need to write a bunch of sequences, listen to them, and figure out if they're usable. And probably to review them now and again, as they can be useful tools to keep in mind.
At least one sequence is fairly well known, though most listeners will probably go "ah, Pachebel's Canon" or "ugh! Pachebel's Canon!" or "(ah|ugh) that distinctive classical piece!" without knowing that the bass uses a particular sequence.
With a longer set of intervals that "go somewhere" you probably instead have a spiral canon. These repeat like a sequence but take more than two steps to get around to the next repeat, and there may be more complicated counterpoint between the voices than in a sequence.
But enough blabbering! Here's some lilypond files.
It may be fun, however, to derive these sequences (or others) from root principles, which here might involve knowing enough about harmony and counterpoint to be dangerous. Certainly one could experiment with oddball sequences (+6 minus something?) though if too unbalanced the sequence may not be able to "walk" for very long. Does it need to, though?
tags #composition