Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:6/8
K:Bb
['F/F/ FFFFGA B2 G G2 G GGGBAG']  
X:1
L:1/8
M:6/8
K:Bb
 F/F/ | FFFFGA | B2 G G2 G | GGGBAG | F2 B D2 F | F2 E/E/ C2 B/B/ | B2 ddBB | c2 c/c/AGA | B4 z |]  