ARRIVING E145 AT LGA OVERSHOOTS TURN ON CHARTED VISUAL APCH.

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WE WERE FLYING FLT ABCD TO LGA ON FEB/WED/03. THE APCH IN USE AT LGA WAS THE EXPRESSWAY VISUAL RWY 31; AND WE WERE CLRED TO CROSS THE DIALS FIX AT 2500 FT; AND CLRED THE APCH TO THE RWY. THE FO WAS THE PF; AND AS IT WAS HIS FIRST TRIP OFF OF IOE AND HIS FIRST TIME EXECUTING THIS APCH AT NIGHT; I WAS TRYING TO GIVE HIM 'TIPS' ON HOW TO MAKE THE APCH (IE; CONFIGURING THE PLANE AND DSCNT RATES; ETC). HE WAS DOING FINE; BUT WITH TRYING TO PAY ATTN INSIDE AND OUTSIDE I MISJUDGED OUR POS TO THE LONG ISLAND EXPRESSWAY Z(LIE); THE REF TO FOLLOW AFTER XING DIALS; AND WE BEGAN OUR DOWNWIND LEG EARLY. THIS EARLY TURN; ADDED TO THE STRONG NW WINDS THAT EVENING; KEPT US FURTHER S OF THE LIE THAN DESIRED GIVEN THE CLOSE PROX OF THE JFK ARPT. ALMOST IMMEDIATELY THE LGA TWR CTLR CAUGHT OUR ERROR AND DIRECTED US TO FLY A 040 DEG HDG AND TO MAINTAIN OUR PRESENT ALT OF 2200 FT. WE DID SO. THE CTLR POINTED OUT TFC WE WERE FOLLOWING AND WE PROCEEDED WITH THE APCH AND LNDG UNEVENTFULLY. BEFORE I LEFT THE ARPT THAT NIGHT; I PHONED THE LGA TWR CAB JUST TO MAKE SURE WHAT IF ANY REACTIONS THERE MIGHT BE TO OUR ERROR; AND TO APOLOGIZE FOR SAID ERROR. THE CTLR WAS POLITE AND UNDERSTANDING OF THE EVENT. THE MAJOR FACTOR TO THE ERROR WAS THE MISJUDGING OF OUR TURN AFTER DIALS. ALTHOUGH I HAD SHOT THIS PARTICULAR APCH SEVERAL TIMES; I BELIEVE TRYING TO HELP OUT MY FO GOT MORE ATTN THAN WHERE WE WERE OUTSIDE. IN THE FUTURE; I THINK I WILL TRY TO FIND OUT IF A NEW FO HAS SHOT THIS PARTICULAR APCH BEFORE; AND IF SO; IF IT WAS AT NIGHT. IF HE/SHE HAS NOT SEEN THE APCH DURING THE DAY; I'LL PROBABLY TAKE THAT LEG AND EXPLAIN THE 'TIPS' AS I DO IT.