For instance if you start a scale on the 2nd note of the C major scale you will be playing “D dorian mode”.
THEN WHY DO PEOPLE FIND IT CONFUSING? HEY, MR JAZZ MAN!
That is because of “transposition”.
We can transpose that “Dorian mode of D” to any tonic we desire.
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.
Below is a Clear explanation and diagram in music notation. Or for my free modes ebook CLICK HERE