ACR LGT TWIN STARTS ABORTED TKOF THEN CONTINUES TKOF AFTER FLAPS ARE EXTENDED. FORGOT CHKLIST USE. NEW CHKLISTS PROCS.

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CLRED FOR TKOF ON RWY 13 AT LGA. FINAL CHKLIST ITEMS COMPLETED; ORALLY REVIEWED INITIAL DEP PROC. CAPT APPLIED THRUST FOR TKOF. WE HEARD A WARNING HORN AND OBSERVED A CONFIGN MESSAGE ON EICAS. CAPT PULLED THROTTLES TO IDLE AS WE BOTH SIMULTANEOUSLY DISCOVERED THE FLAPS STILL RETRACTED. CAPT CALLED FOR FLAPS EXTENDED. I HESITATED; THINKING WE WERE GOING TO PULL OFF THE RWY. CAPT AGAIN SAID TO EXTEND FLAPS; THAT WE WOULD BE OK. I COMPLIED; EXTENDING FLAPS TO 5 DEGS (OUR PLANNED TKOF SETTING) AND WE RESUMED THE TKOF ROLL. LIFTOFF AND CLB WERE NORMAL EXCEPT ON CLBOUT AS I WAS TRYING TO PIECE TOGETHER WHAT HAD TAKEN PLACE I REALIZED TO MY HORROR THAT MY INITIAL ERROR OF LETTING MYSELF BECOME DISTRACTED AND MISSING THE FLAP EXTENSION AFTER ENG START HAD BEEN COMPOUNDED BY OUR MISSING COMPLETELY THE INITIAL ITEMS ON THE 'BEFORE TKOF' CHKLIST. THIS; OF COURSE; WOULD HAVE BEEN OUR 'BACKUP' SAFETY CHK. WE REVIEWED AND DISCUSSED THE EVENTS LEADING UP TO AND OUR HANDLING OF THE INCIDENT THROUGHOUT MUCH OF THE REMAINDER OF THE FLT. I HAVE REPLAYED IN MY MIND OVER AND OVER AND OVER EVERYTHING I DID FROM THE TIME I GOT UP THAT MORNING ALL THE WAY THROUGH THE FLT. PICKING IT UP AT THE PUSHBACK: WE HAD BEEN DELAYED AT THE GATE DUE TO SOME OTHER TFC OUT AND INBOUND. WE WERE PUSHED BACK ONTO THE INNER TXWY WITH CLRNC TO START ENGS DELAYED UNTIL BRAKES WERE SET. WE RECEIVED A SALUTE AND RELEASE FROM THE MECH AS I WAS STARTING THE L ENG. ENG STARTS WERE NORMAL AND CAPT CALLED FOR TAXI CLRNC. SINCE WE HAD BEEN BLOCKING THE INNER I FELT A BIT RUSHED AND CALLED GND WHILE STARTING MY AFTER START FLOW. THAT FLOW WAS THEN INTERRUPTED AS WE RECEIVED TAXI INSTRUCTIONS. THE CAPT BEGAN TO TAXI; I CHKED FOR WING CLRNC; CHKED THE TAXI CHART TO UNDERSTAND THE RTE. THE CAPT THEN SAID HE WAS CHKING THE RUDDERS AT WHICH TIME I THEN CHKED AILERON AND ELEVATOR MOVEMENT SO AS NOT TO FORGET. I THEN NOTICED A FLASHING MESSAGE INDICATION ON THE ACARS. IT WAS OUR UPDATED WT MANIFEST SO WHILE DIVIDING TIME BTWN INSIDE AND OUTSIDE I CHKED PAX COUNT; INSTALLED THE NEW ZFW IN THE FMC AND RESET TRIM; CHKED THE NEW WT AGAINST THE CARD AND ADVISED THE CAPT OF THE NEW TRIM AND V SPDS. BY THIS TIME WE WERE APCHING OUR HOLD SHORT POINT FOR XING RWY 4. I HAD COMPLETELY FORGOTTEN THE REST OF MY AFTER START FLOW; WHICH; OF COURSE; INCLUDED EXTENDING THE FLAPS. AS WE WERE HOLDING SHORT OF RWY 4; I THOUGHT OF THE 'BEFORE TKOF' CHKLIST BUT REJECTED THE IDEA AS I WAS AFRAID; IRONICALLY; THAT WE WOULD BE INTERRUPTED BY A CLRNC TO CROSS THE RWY AND THOUGHT IT WOULD BE SAFER TO WAIT UNTIL AFTER WE HAD CROSSED. AFTER XING WE WERE MORE INVOLVED IN WATCHING THE TAXI RTE AND WITH OUR ATTN DIVERTED COMPLETELY OUTSIDE; NEITHER OF US REMEMBERED THE CHKLIST. SOMEHOW IN THE LATER STAGES OF TAXI AND WAITING FOR TKOF CLRNC; WE HAD FAILED TO PROVIDE A CUE TO REMIND US ABOUT THE CHKLIST. CURIOUSLY; AS I MENTIONED BEFORE; WHEN TKOF CLRNC WAS RECEIVED; WE WERE BOTH CUED TO DO THE FINAL ITEMS (BELOW THE DASHED LINE ON THE CHKLIST) AND REVIEW THE TKOF PROFILE BUT HAD NO CLUE TO THE FACT THAT WE HAD FORGOTTEN THE INITIAL ITEMS. SOME OTHER RELEVANT FACTORS WOULD BE: 1) MY INEXPERIENCE -- ABOUT A MONTH ON THE AIRPLANE. 2) NOT PRIORITIZING MY WORKLOAD PROPERLY; LETTING MYSELF TAKE THINGS OUT OF SEQUENCE. 3) NOT GIVING MYSELF AN EXTERNAL CUE AS A REMINDER FOR THE CHKLIST; SUCH AS PERHAPS PULLING THE CHKLIST OUT OF ITS SLOT AND PLACING IT ON A CLIPBOARD. 4) SOMEWHAT UNFAMILIAR WITH THE ARPT AND TXWYS. SOME OTHER FACTORS MIGHT BE: TYPICAL HIGH STRESS; NOT A VERY GOOD NIGHT'S SLEEP IN DOWNTOWN MANHATTAN; BRAND NEW CHKLISTS (DIFFICULT ITEMS AND SEQUENCES) THIS MONTH. AS A CREW WE HAVE DONE DEEP SOUL SEARCHING AND ARE PARTICULARLY DISTRESSED THAT WE COULD HAVE ALLOWED OURSELVES TO BECOME SO CARELESS. NEITHER OF US IS COCKY OR ALOOF IN OUR JOB; BUT STRIVE TO BE CONSCIENTIOUS AND ARE BIG BELIEVERS IN SOP. WE READ THE ACCIDENT/INCIDENT AND SAFETY RPTS AND TRY TO APPLY THE LESSONS LEARNED BY OTHERS. WHICH LEAVES US WONDERING -- 'HOW COULD WE LET THIS HAPPEN TO US?!'