In the example below it is transposed to C as the tonic. 

How did I do this?

Well if the note D [Tonic of D dorian] is two 1/2 steps up from C then what is the note C two 1/2 steps up from? Answer is B flat.

So we have a B flat scale staring on the note C.

Transposed modes with C as Tonic

Video Explanation

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