C minor 7th with flattened fifth, for example, comprises these notes:
Eb - Gb - Bb - C
So now, when I'm asked to play Dm7(-5), I actually play Fm6; when I'm asked to play Em7(-5), I actually play Gm6; and so on.
It's a simple little trick that I stumbled on recently. If you know much about music, you may be surprised that I was unaware of something so 'basic'.
But it has been a helpful discovery to me; and I hope it may benefit some readers.
For an example of this chord in action, think of the opening line of that lovely old song from 1913 'If I had my way, dear, forever there'd be a garden of roses for you and for me....'. The word way is sung with the chord of the minor 7th with flattened fifth.
And still further thoughts have been sent to me by my old friend John Burns: