LGT FLC DSNDS BELOW ASSIGNED PRIOR TO REACHING INTXN. WRONG VOR TUNED.

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ON RADAR VECTORS AT 4500 FT; TOLD TO EXPECT LDA DME RWY 30L; TO INTERCEPT LOC. THEN CLRED FOR ILS RWY 30L; MAINTAIN 4500 FT UNTIL FONTI. I CHANGED #2 NAV TO ILS RWY 30L AND VERBALLY STATED SO. I THOUGHT #1 NAV WAS TUNED TO STL VOR. I ANNOUNCED DME 22.4 STL; AT FONTI; DSNDING TO 3500 FT. #1 DME WAS ACTUALLY TUNED TO LDA DME RWY 30L. AT 4000 FT; CAPT ANNOUNCED I STILL HAVE #1 TUNED TO LDA DME RWY 30L; YOU ARE NOT TO FONTI YET. I LEVELED OFF AT 4000 FT THEN INTERCEPTED GS AND SHOT APCH. I THINK SOME CONTRIBUTING FACTORS ARE: APCH CTL BEING VERY BUSY; CHANGING APCH AT LAST MIN; FLT DECK CREW BEING IN BUSY PART OF FLT. THE CAPT NOT STATING WHEN CHANGING #1 NAV/DME; MYSELF NOT VERIFYING #1 NAV/DME SELECTION. HAD I CHOSEN TO DSND WHEN INTERCEPTING THE GS AT 4500 FT; THE PROB COULD HAVE BEEN AVOIDED. IT WAS OUR FIRST DAY FLYING TOGETHER; I FEEL IF OUR COM LEVEL WAS BETTER; THE PROB COULD HAVE BEEN AVOIDED. I HAVE ALSO SEEN IN MANY CASES; A LOT OF LAST MIN CHANGES IN CLRNCS CAUSING CONFUSION MAINLY DUE TO TIME CONSTRAINTS AND COM ERRORS. THERE WERE NO ACFT BELOW US; VFR CONDITIONS EXISTED; THE ALTDEV DID NOT CAUSE AN ACFT CONFLICT.