I'm struggling to get from E Minor to a Bbsus chord. Once I get there, my modulation to C minor is good but I'm struggling to find something that moves me smoothly to the Bbsus. Any thoughts?
I don't know why you need to use Bbsus, but I can think of two other ways you can modulate.
First, a way to modulate from E minor to C Minor is chromatically. Very simple, by flatting the E and B. Em is E,G,B and Ebm (III of C minor) is Eb,G,Bb. So, by this chromatic modulation, you find yourself to the C minor scale. *
And secondly, while on E minor, you can play Em (i), G(III) which G is the V of C minor and simply play Cm afterwards.
These two tonalities are a bit afar; E minor has one sharp while C minor has three flats, so the two aforementioned methods might sound a bit off, but they are the quickest way there.
A more smooth, but longer way would be to follow the circle of fifths leftwise. E minor to A minor to D minor to G minor to C minor, but that would take more time.
Since you like the sound of Bb sus, you can approach it chromatically. Similarly to the first way described above, you can play Em (E,G,B) and flat the E and B to Eb and Bb respectively, remove the G and you have the Bbsus chord. This might not be really smooth. Another way would be to play some other chord from the E minor scale that contains either the note E or the note B.
*Generally, chromatic modulations are used to approach tonalities that far apart.
Everything depends on how long there is to make the transition, the harmonic rhythm within that time, and the demands of whatever might be happening melodically. That is to say: how many chords do you have time for, how long can you give a listener to adjust to the new harmony, and what restrictions does the melody place?
A general approach, though, might be to construct the progression in reverse. For discussion's sake, the simplest, smoothest way to get to the Bbsus is from a chord with two common tones. Here is a chart of options in which the non-common tone is within a semitone of its destination:
D-F-Bb potentially gives away the game (= BbMaj) E-F-Bb very dissonant Eb-E-Bb very dissonant Eb-Gb-Bb Ebmin, the IV chord of the destination Eb-F-A basically F7, so the V chord of the destination Eb-F-B a more dissonant version of the F7 chord
This approach could be extended all the way back to the starting Emin.
Here are a couple of options that use that approach in part, but also rely on just messing around with what sounds good (to my ear).
Emin -> GMaj -> Gmin -> EbMaj -> EbMin -> Bbsus
X: 1 T: Emin -> Bbsus #1 K: none M: none L: 1/1 [EGB] [DGB] [DG_B] [_EG_B] [_E_G_B] [_EF_B]
Emin -> Eb+ -> GMaj -> AMaj -> Dmin -> F7 -> Bbsus
X: 1 T: Emin -> Bbsus #2 K: none M: none L: 1/1 [EGB] [_EGB] [DGB] [^CEA] [DFA] [_EFA] [_EF_B]
By the way Bb sus 'what' - sus2/sus4 ??? - And which music style ? 'Jazzy' with a lot of tensions allowed or more 'Classical', 'Pop' ? - How many bars are available - slow or quickest possible modulation??? And above all 'for which instrument' - solo or arrangement/orchestration?
- For a quicky from 'Em' to 'Cm' on the Piano just go 'Barbra Streisand':
| Em Em | F/G G || Cm(add9) | ...
The highest notes also
| e e | f g || g | ...
With the progression IV/V - V - I respectively IV/V - V - i (as seen from your new tonic) you can go absolutely everywhere in one bar. Major or Minor! Em is already part of C - you just decide in the last moment to go 'minor'... ;-)