B757-200 CAPT REPORTS BEING OFF COURSE AND UNSTABILIZED DURING EXPRESSWAY VISUAL TO RWY 31 AT LGA.

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A REVIEW OF THE PREDEP WX INDICATED THE APCH IN USE WOULD BE EXPRESSWAY VISUAL 31. WHILE LOADING THE FMS; I WAS UNABLE TO SELECT THE APCH. I FOUND A NOTE ON THE FLT PLAN AND 'BUILT' THE APCH MANUALLY. ABOUT 1 HR OUT OF LGA; FO FOUND INSTRUCTIONS TO CREATE THE APCH ON OUR FMC. WHILE ON APCH; TWR INSTRUCTED US TO SQUARE OUR TURNS TO FINAL AS THEY WISHED TO DEPART 2 ACFT PRIOR TO OUR LNDG. I OVERFLEW THE CTRLINE AND CAME BACK TOWARD THE CTRLINE. AFTER LINING UP; FO ADVISED THAT I WAS GETTING LOW. CORRECTED AND MADE A NORMAL LNDG IN THE TOUCHDOWN ZONE. I DON'T FLY TO LGA THAT MUCH AND I HAD NEVER HEARD THESE INSTRUCTIONS BEFORE. I WAS UNSURE OF WHAT THEY WANTED AND WHILE CIRCLING I WAS OVERLY RELIANT ON VNAV PATH. THE APCH FELT RUSHED AND UNPROFESSIONAL. I SHOULD HAVE DECLINED THE INSTRUCTIONS AND FLOWN THE NORMAL APCH. I HAD DONE AN APCH BRIEFING PLANNING THAT I COULD USE OUTSIDE REFS FOR PITCH GUIDANCE; BUT STILL FAILED TO USE THE VASI AS MUCH AS I COULD HAVE. IN SHORT; EVEN THOUGH I PLANNED THE APCH BEFORE I LEFT; IT FELT LIKE I NEVER CAUGHT UP.