Mat 23:2 Saying, The scribes and the Pharisees sit in Moses' seat:
As we all know, Moses' seat, although not a chair, it is the position of authority over the Jewish people...Whatever came out of Moses' seat was to be authentic Jewish teaching, accepted by the religious Jewish community over the course of many years, decades or centuries...
These people were in charge of overseeing the oracles of God as given to the Jews by God...
Mat 23:3 All therefore whatsoever they bid you observe, that observe and do;
The idea is that whatever the Pharisees bid you do is the same as what was given by the Scribes and Pharisees thruout history...The oracles don't change from one group of Pharisees to the next...
but do not ye after their works: for they say, and do not.
But the idea in the context as I see it is that this group of Pharisees are reading the actual oracles and maybe going well overboard on their (the oracle's) intent, all the while doing things far differently that the oracles prescribe...
Mat 23:4 For they bind heavy burdens and grievous to be borne, and lay them on men's shoulders; but they themselves will not move them with one of their fingers.
Mat 23:5 But all their works they do for to be seen of men: they make broad their phylacteries, and enlarge the borders of their garments,
It's clear to me that Jesus told these folks to follow what the Pharisees are telling you as long as it's word for word out of historical Moses' seat...Ignore their embellished version and definitely don't do what you see them do whatever it is, that is contrary to historical teaching...