Write out all the notes between C and B. They are C, C#, D, Eb, E, F, F#, G, Ab, A, Bb, B.
Assign each note a number 1-12. In this case: C=1, C#=2, D=3, Eb=4, E=5, F=6, F#=7, G=8, Ab=9, A=10, Bb=11, B=12.
Write out all of the notes between Bb and A. It will look like this: Bb, B, C, C#, D, Eb, E, F, F#, G, Ab, A.
Assign each note a number. It will look like this: Bb=1, B=2, C=3, C#=4, D=5, Eb=6, E=7, F=8, F#=9, G=10, Ab=11, A=12.
Look at the music that you are transposing from your root, "C." Practice playing one bar of your song. For example, let's say it goes: C, G, F, E, D, C', G. This is the beginning of John William's "Star Wars" theme.
Assign each note you just played a number. 1, 8, 6, 5, 3, 1, 8
Match these numbers with the numbers that correspond to your Bb scale. Bb, F#, Eb, D, C, Bb', F#
Match the remaining notes in your song to the notes in the key of Bb.