Exposure Triangle
The Exposure Triangle is a handy mental model to help with manual exposure strategy. I first decide which corner of the triangle to “lock down” first. If there is fast movement (e.g. bird in flight) then I set the shutter speed to 1/1600 sec or faster. If there is a large object, or the object is very close to me, then the depth of field is most important, so I lock down the aperture corner first. If I set shutter speed first, next I think about depth of field relevant to the composition, set the relevant aperture, and lock that down.
Once I lock down the shutter speed and aperture I spot meter on a grayish part of the scene (rock, gravel, fence post) and adjust the ISO to center the exposure. I then leave things as they are set and start photographing.
If the light is plentiful then I have more room to increase the shutter speed in a wildlife action scenario, if needed; or in a landscape scenario I may want to stop down the aperture to improve depth of field. It is much easier when there is a ample light.
The challenges occur when the light is low. This is when manual exposure really helps. If I am underexposed on the light meter for ISO values less than < 1600 then I have to be careful I do not amplify noise instead of signal. I might decide to relax my ideal settings for shutter speed and aperture to increase the true exposure, before I start to crank up the ISO. Perhaps for the birds in flight I am willing to fully open up the aperture and not worry about depth of field as much, or instead of shooting at 1/1600 sec I am willing to go down to 1/800 second shutter speed. Manual exposure allows me to make these tradeoffs in a controlled, knowable, manner.
Most important though I ask myself what is more important to me for the image I am working on. Perhaps I am not willing to lower the shutter speed or the open up the aperture. I will then raise the ISO and rely up on using de-noising methods in post-processing. Uniform areas (sky, water, snow) are pretty easy to clean up noise on. Areas of the composition with detail that you want to retain (fur, feathers, grass, foliage) sharpness can be degraded in post-processing de-noising.
It is better to have a non-blurry noisy image, than a blurry image with less noise.