First things first. Search for other demos of the same product. Watch them. If someone else has already done a demo you can't improve on, don't do another one.
Second thing: Decide who your audience is. Is it beginners? Working professionals? People who play in a specific genre? That determines what you need to explain. Beginners may need to know how to properly connect the pedal to their bass and amp. On the other hand, it would be an insult to include that in a demo for professionals unless there was something unusual about it.
Third thing: Tune your bass. Make sure the intonation is good. If you can't play in tune don't do the demo.
Fourth: Know what the pedal does. Practice with it ahead of time. Read the manual. Figure out which settings you're going to use. Make an outline of the points you want to cover before you turn the camera on.
Fifth: Tune your bass again.
Sixth: Set the audio levels of your off camera mic and direct bass signal.
Seventh: Adjust your lighting, focus the camera, turn your phone off, make sure the dog and the baby are fed and taking a nap in the other room.
After all that, you're ready to start recording.
Make sure you include samples of the dry sound of the bass. That's the only way people can know exactly what the pedal is doing.
Show the knob settings every time you change them. Picture in picture is your friend. Tell the viewer what you're doing. Briefly. "Now I'm going to turn the gain up and the master volume down." "If you turn the rate knob all the way down you get a good funk tone when you play near the neck, like this." "With the EQ at 3:00 the low B starts to distort."
Play parts of songs that demonstrate what the setting you're using would actually be used for. If it's a good slap tone, slap. If it's a good low groove tone, play a low groove. If it sounds like Entwhistle, play a Who song. The most boring thing in the world is when a demo is 16 bars of identical playing for each of 10 different settings.
Last thing: After you've finished editing the video, watch it back and ask yourself if it really is better than the other demos you watched in step 1. If not, don't post it.