Help with exposure control please...
For example, I want to have my subject matter underexposed, whilst the background is still relatively light
You say you're experimenting with Manual mode (and why not?), so here's what I suggest:-
Keeping the camera in M mode, expose for the bright area, i.e point the camera at the bright area and change the settings so that the exposure indicator in the viewfinder shows a good exposure.
And that's basically it! The bright areas will now be properly exposed and your subject will be too dark. To get a hint of light on your subject, get them to turn their faces towards the light. You are in M mode, so it's east to brighten or darken the image to suit. This is a bit simplistic as of course it depends on the various circumstances and light and where the subject is placed.
When you think about it, although we all call the M mode 'manual', its really semi-automatic as you're shown the exposure through the viewfinder, so really you can take advantage of that.