INSTRUCTOR WITH STUDENT FLIES THROUGH LOC AND BELOW ASSIGNED ALT ON ILS APCH.

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I AM AN INST FLT INSTRUCTOR; WAS INST CURRENT AND HAD CURRENT IFR CHARTS WITH ME AT THE TIME OF THIS OCCURRENCE. I WAS GIVING DUAL INSTRUCTION TO ANOTHER FLT INSTRUCTOR WHO FLEW IN THE L SEAT. WE HAD FLOWN FROM W TX; TO LIT; VFR ON TOP OF A CLOUD LAYER EXPECTING; ACCORDING TO FORECASTS; SCATTERED TO BROKEN CLOUDS OVER LITTLE ROCK. ABOUT 20-30 NM W OF LITTLE ROCK IT BECAME APPARENT THAT THE CLOUDS WERE NOT BREAKING AS FORECAST. I CONTACTED LITTLE ROCK APCH CTL AND ASKED FOR AN IFR CLRNC TO DSND THROUGH THE CLOUD LAYER. APCH PROMPTLY GAVE US A CLRNC AND VECTORS TO THE ILS 22R APCH. FURTHER CLRNC INSTRUCTED US TO MAINTAIN (I BELIEVE) 2000 FT UNTIL WE WERE ESTABLISHED ON THE LOC. I WAS ATTEMPTING TO TALK TO THE OTHER PLT; WHO WAS UNFAMILIAR WITH THE ACFT; THROUGH THE PROC. IN THE TURN TO INTERCEPT THE LOC WE BROKE OUT THE BOTTOM OF THE CLOUD LAYER. WE FLEW THROUGH THE 22R LOC TO A POS WHICH APPROXIMATED A FINAL APCH TO RWY 22L. WE WERE CORRECTING BACK TO THE 22R LOC WHEN APCH CTL QUERIED AS TO OUR LOC FREQ SETTING AND ORIENTATION. MEANWHILE WE HAD CONTINUED TO DSND TO ABOUT 1500 FT; WELL BELOW THE GS; WITHOUT ACTUALLY HAVING ESTABLISHED OURSELVES ON THE LOC. WE DID FINALLY ESTABLISH A STABILIZED VISUAL APCH WELL BEFORE ACTUALLY REACHING THE RWY. SOME FACTORS WHICH I BELIEVE CONTRIBUTED TO THIS OCCURRENCE: 1) MY FAILURE TO REMEMBER THAT THE OTHER PLT WAS UNFAMILIAR WITH THE ACFT. THIS UNFAMILIARITY INCLUDED AIRSPDS AND PWR SETTINGS APPROPRIATE FOR INTERCEPTING AND FLYING THE APCH. 2) MY FAILURE TO TAKE OVER OP OF THE ACFT WHEN IT BECAME OBVIOUS THAT THE OTHER PLT WAS GETTING INTO DIFFICULTY IN SPITE OF MY ATTEMPTS TO HELP HER THROUGH THE PROCESS. THIS SITUATION HAS MADE IT VERY CLR TO ME ONCE AGAIN; THAT AS FLT INSTRUCTOR ON A FLT; I AM ALSO PIC AND AS SUCH HAVE THE RIGHT AND RESPONSIBILITY TO MAINTAIN CTL OF THE ACFT AT ALL TIMES.