You need video editing software, and audio editing software. Depending on your audio interface, you may already have something you can use.
I used Studio One for my DAW. It recorded 7 tracks in 48kHz/24 bit audio through a PreSonus StudioLive mixer. I mixed it down to a 48kHz/16 bit stereo WAV file.
The iPhone video had to be emailed to me from my phone because Apple iOS won't let me access the data via USB connection.
Once I had the video on my computer, I imported it into video editing software (Adobe Premiere Pro, but I could've used iMovie). In that program, I imported the stereo mix. I used the audio signal wave forms to synch the sound. Once it was synch'd (in about 10 seconds), I muted the iPhone audio and exported the video to the YouTube video format. BTW, the YouTube video format uses 48kHz/16 bit audio, which is nice.
This entire mixing/editing process took about 30 minutes.
I don't use an iPhone any more 'cuz I don't want to deal with extracting the video from my phone. I use a used Canon D70 set to basic 1080p video recording and import the video from an SD memory card. Much faster than dealing with an iPhone.
That video, and many others were recorded in a space I'm no longer in. It was a large room with concrete floor and 24' high ceilings. The reverb was extraordinary. To partially tame it, I hung moving blankets around my kit. One lesson from recording in that space: the room greatly affects the sound of the drums. A smaller, tighter room would not have let those toms sound so open, in my amateur observations.
⬇ This is a similar setup but with an added 14" tom.
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