Keep it simple, stupid.
For Senior Photos and Engagements I get to choose what time to begin the session, so I only schedule for when all the elements are in my favor. I begin every single photo session ~2 hours before sunset. At this time of day, there's plenty of light out, but the sun is lower is the sky so it won't cast unflattering shadows on my subject's face.
My process for choosing aperture/shutter speed/ISO for these situations stays constant. I don't reinvent the wheel every time I shoot. Keep in mind that there isn't a "right" or "wrong" when it comes to art, but this is just how I choose my settings:
1. Set my ISO at 100: Since I always schedule shoots during the day when there's plenty light outside. That way I can keep my ISO low as possible. Grain isn't my thing. 100 is the lowest ISO option, but there is definitely no noticeable difference in grain level as long as you're under 400. Especially if you invest in a full frame camera.
2. Determine what Aperture I want: I like to keep my aperture low since I really like blurry backgrounds. Here is what aperture I typically choose depending on what I'm shooting:
1 person (e.g. senior photos) : ~ f/1.8
2 people (e.g. engagement sessions) : ~ f/2.2
A group of people in 1 line (e.g. bridesmaids) : ~ f/2.8
A group of people in 2 rows (e.g. bigger bridal parties) : ~ f/4
3. I take a test photo and see how light or dark it is
4. I change shutter speed depending on how light or dark the test photo was
Test photo was too light —> faster shutter speed
Test photo was too dark —> slower shutter speed
Examples:
Aperture: f/2
Shutter Speed: 1/1600
ISO: 100
Aperture: f/2.5
Shutter Speed: 1/500
ISO: 125
Aperture: f/2.8
Shutter Speed: 1/350
ISO: 100
