Summary: Use OBS, Camo and Audacity.
I recently needed to record a presentation which had slides and my face combined, using a Mac. Based on suggestions from friends and searching the web, I came up with a recipe that worked reasonably well. I'm writing this down to both share that recipe, and so I can reuse the recipe next time.
Slide design: I used a slide template which had a vertical rectangle hole at the bottom left so I could overlay a picture of my video. It took a while to find a slide design that looked plausible, and make sure callouts/quotes etc didn't overlap into this area.
Camera: The best camera you have is probably the one on your phone. To hook up my iPhone to my Mac I used a £20 lightning to USB-C cable (next day shipping from Apple) along with the software Camo. I found Camo delightfully easy to use. I paid £5 per month to disable the logo and because I wanted to try out the portrait mode to blur my background - but that mode kept randomly blurring and unblurring things in the background, so I didn't use it. Camo is useful, but I record videos infrequently, and £5/month is way too steep. I'm not a fan of software subscriptions, so I'll remember to cancel Camo. Because it is subscription based, and subscribing/cancelling is a hassle, I'll probably just suck up the logo next time.
Composition: To put it all together I used OBS Studio. The lack of an undo feature is a bit annoying (click carefully), but otherwise everything was pretty smooth. I put my slide deck (in Keynote) on one monitor, and then had OBS grab the slide contents from it. I didn't use presentation mode in Keynote as that takes over all the screen, so I just used the slide editing view, with OBS cropping to the slide contents. One annoyance of slide editing view is that spelling mistakes (and variable names etc.) have red dotted underlines, so I had to go through every slide and make sure the spellings were ignored. Grabbing the video from Camo into OBS was very easy.
Camera angle: To get the best camera angle I used a lighting plus phone stand (which contains an impressive array of stands, clips, extensions etc) I'd already bought to position the camera right in front of me. Unfortunately, putting the camera right in front of me made it hard to see the screen, which is what I use to present from. It was awkward, and I had to make a real effort to ensure I kept looking into the camera - using my reflection on the back of the shiny iPhone to make sure I kept in the right position. Even then, watching the video after, you can see my eyes dart to the screen to read the next slide. There must be something better out there - or maybe it's only a problem if you're thinking about it and most people won't notice.
Recording: For actual recording there are two approaches - record perfectly in one take (which may take many tries, or accepting a lower quality) or repeatedly record each section and edit it together after. I decided to go for a single take, which meant that if a few slides through I stumbled then I restarted. Looking at my output directory, I see 15 real takes, with a combined total of about an hour runtime, for a 20 minute talk. I did two complete run throughs, one before I noticed that spelling mistakes were underlined in dotted red.
Conversion to MP4: OBS records files as .mkv, so I used VLC to preview them. When I was happy with the result, I converted the file to .mp4 using the OBS feature "Remux recordings".
Audio post processing: After listening to the audio, there was a clear background hum, I suspect from the fan of the laptop. I removed that using Audacity. Getting Audacity to open a .mp4 file was a bit of an uphill struggle, following this guide. I then cleaned up the audio using this guide, saved it as .wav, and reintegrated it with the video using ffmpeg and this guide. I was amazed and impressed how well Audacity was able to clean up the audio with no manual adjustment.
Sharing: I shared the resulting video via DropBox. However, when sharing via DropBox I noticed that the audio quality was significantly degraded in the DropBox preview on the iOS app. Be sure to download the file to assess whether the audio quality is adequate (it was fine when downloaded).
