*** I hope others will add comments here.
The goal: to take videos of some partly inaccessible software to send to its developer. The videos would need to be quick clips that only show the problem; not long-winded amateur off-the-cuff recordings that would bore the developer.
The problem: blind with just light perception and no experience of being sighted before. No idea how to do this.
The solution: First, practice with the software, making audio recordings for your own use if you need to keep track of the steps you took. Get to where you can demonstrate the problem in under 90 seconds. For example "on this screen labeled enter customer data, I can tab to the first and last name fields fine, but there's no way to get to the address or phone number field with the keyboard".
Second: now you got your spiel down, get yourself a cheap tripod for the phone. There are big ones that are four or five feet tall, but a little one which securely sits on your desk can be had for around $10 U.S. If you need to make it work better, (my monitor is on a stand) you can always stand the mini-tripod on a stack of books.
Third: if your software has a high-contrast mode turn that on. The iPHONE wants to autofocus on the brightest objects. Be sure to maximize the window you are working in so it fills the entire screen.
Fourth: Position the tripod where you think you need it. Remember the camera is on the upper right corner of the phone. So it faces the center of the computer monitor while the phone faces you. Most modern monitors are in landscape mode, where the aspect ratio is 9:16, whereas older monitors are portrait, 4:3. Read about aspect ratio here:
Now you think your camera is correctly oriented, pull up any screen loaded with text that your screen reader can read just fine. Then run some OCR apps. I like the free Seeing AI or the KNFB reader for their field of view reports. Once they can detect the edges of the screen and correctly OCR all the text, you know the camera is seeing your computer monitor fine. You will have to move the phone back and forth, up and down and possibly tilt the monitor. Do this in a dark room with the screen brightness high. Remember to turn the screen curtain off or the camera doesn't work correctly on the phone. I found my iPHONE needed to be about a meter or yard away from the monitor. If you can't get this part to work, you'll need a sighted person to help. I was able to get reasonable videos with no sighted help; better ones when help was available.
Fifth: using painters tape or anything which won't leave a stain or sticky residue, mark the place on your desk where the tripod and/or the monitor must be position for proper viewing. Now make some practice videos of screens you can easily access with your screen reader. Adjust the volume of your speakers until the video sounds good, because of course you want the developer to hear what your screen reader does and does not see. You might need to practice until your voice and the screen reader are at the same volume.
Sixth: share the videos with sighted friends. Respond to their feedback. Can they hear the screen reader OK? Do you need to slow it down or change to a higher-quality voice? Do they see the dialog box clearly or is part of the screen cut off? For example, if the screen seems fuzzy you can experiment with turning off auto-focus and ae exposure. This video:
is particularly clear about how all the hidden camera features work.
One tip, if you need to force focus on a particular part of the screen, double-tap there. If you can't see where to do this in the viewfinder, simply experiment and run your resulting photos through an OCR program like KNFB reader or Seeing AI to find out if they still read OK. To lock the focus so that movement won't trigger the auto-focus, double-tap and hold. These are of course both gestures for voiceover, and not for the sighted user who only needs to single tap or single-tap and hold.
Seventh: Hook up your headphones so you can push the volume button to start the recording. You can double-tap the "take video" button of course but that may shift the iPHONE's position a bit. Better to do it remotely.
Eighth: Now that sightlings say your video is acceptable, run through your spiel with the inaccessible parts of your software. Make several videos and talk during all parts letting your screen reader read what it can.
Ninth: OK, time to edit. In the photos app, choose your video, choose edit and adjust the scrubbers to cut out the parts you don't want.
There is a great podcast here on trimming videos, and also simply exploring the camera app will help you learn the ins and outs of video recording. Remember with stable objects like a computer monitor, your results can be as good or better than those of sighted people.