We had a lot of fun with this. We first looked at getting someone to do it for us but it was way expensive. So we decided to write a script, brainstorm frames and got one of our colleagues to do it in - house
Canva is great but we've also started using Loom/Cloudapp for walkthroughs. in the same line of development Arcade is a great tool for walkthroughs if you're not using a product like intercom that has Product Tours built in native.
Although I may be biased, you definitely need to ensure the audio quality is as good as possible because nothing is more distracting to your audience than poor audio quality.
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Lyrist