"came down from" is less of a literal expression of elevation (Judea being higher in elevation than Antioch), than that of the Jewish world view. The Jews viewed travel to Jerusalem and the temple, as going “up”, and travel away from Jerusalem and the temple as going “down”.
## taught the brothers ##
"continued to teach the believers at Antioch" or "were teaching the believers at Antioch"
## after the custom ##
"according to the custom" or "following the teaching"