Now that my "YouTube" channel has exceeded four hundred subscribers, I feel confident to treat my fellow video makers to a modest lecture on what constitutes good practice when making videos for this medium.
Incidentally, careful observation of successful "YouTubers" shows that the YouTube algorithm seems to reward behaviours that are largely opposite to what constitutes good practice, so if you aspire to be a major "influencer" or "content creator" you will probably need to meticulously avoid all that is advised hereafter.
Get straight to the point and start with your video's substance in the first few seconds. Waste no time, bandwidth, system resources, and energy with an intro, let alone a generic channel intro that is the same in each of your videos. Imagine someone watching all of your videos and having to sit out the same intro every time again! Not only does this waste your viewers' time, it also provokes an enormous amount of annoyance, boredom, and aggression, and generates unneeded exhaust of harmful gasses into the Earth's atmosphere as a result of the extra energy consumption.
Likewise, waste no time with a "preview" of what is to follow. The fact that such previews are typically accompanied by advice as to how to "skip the preview" betrays that viewers do not want a preview altogether, and that the maker is well aware of this. Again, this creates tremendous annoyance, and has all the other disadvantages mentioned under the previous point.
If you are going to convey substantial information in spoken form in your video, prepare your text, making it clear, concise, and relevant. Do not just turn on the camera and start rambling; except if you are a highly skilled improviser or orator, that is.
Unless you are a composer of operas, never put music under spoken text. It only distracts, annoys, and makes it hard or impossible to hear or "catch" what is being said. The viewer can not escape this by turning down the volume because the speech becomes softer too then.
If a video contains both music and speech, the music should not be louder than the spoken text because that would necessitate the viewer to constantly reach for the volume control, turning it down each time the music starts and turning it up every time the speech begins. In general, any kind of external material or clip included in a video should be balanced with the rest of the video in terms of sound volume.
Speak at a normal speed. Do not rattle and then arrogantly write in the comment section, "You can play it at three-quarter speed if it goes too fast for you". The mere fact that such advice is needed betrays that viewers do not want to hear you rattle to begin with. Also, and as you know very well, you recorded the video at your normal talking speed, edited out all the natural speaking pauses, and accelerated it in a video editor to create the impression that you are a really quick-minded person.
Refrain from asking viewers to "like", "share", subscribe, give you a "thumbs up", or "click the notification bell". This only wastes time, especially since that non-information tends to be included in each and every video from a creator thus inclined. Sadly the YouTube algorithm rewards smooth-talking beggars like this, but one should focus on making good videos for a human audience, not cater to an algorithm. It is then up to YouTube to recognize quality and ensure it gets seen.
Refrain from asking viewers to send you money or "support" you on "Patreon" or whatever; if you want to make money, sell your work or get a daytime job. You should not want to generate additional income through YouTube videos; rather you should want to pay YouTube to be allowed to publish your videos free of advertising. Yes, viewers can already pay to be rid of advertisements; but it is the uploaders who should be paying to spare their audience the plague of advertisements. After all, if you were providing your videos through a server infrastructure of your own, it would cost you money too, and even a lot more because you would lack the benefits of scale that a large corporation like YouTube has.
Refrain from using abbreviations or idiomatic (non-literal) expressions. You may assume that everyone knows them just because you know them, but that is not so and results from involuntary and unaware projection of your own mind contents on all others. In actuality, abbreviations and idioms tend to be known to a much smaller circle of people than you think, while your videos can be seen by the entire world population, many of whom have no clue what you are rambling about but are intelligent enough to understand what you are saying if only you speak in proper literal language.
Include no third-party-sponsored material or hidden third-party advertising; if you want to make money, sell your own work.
If you include explicit, open, recognizable third-party advertising ("paid promotion") it should constitute no more than two percent of the duration of your video. That is the maximum amount of advertising that is still bearable. And of course, under no circumstance should advertisements sound louder than the rest of the video. If you allow YouTube/Google to place advertisements in your videos, you have no control over their durations, so draw your conclusions and abstain from "monetizing" your channel. Yes, YouTube puts advertisements in videos even if you do not monetize your channel nowadays, but that is bad practice and as an ethical video maker you should not want to be responsible for that or profit from it.
Include no links to other videos of yours inside the present video, unless possibly at the end of it, and even then very briefly; you must want people to keep watching the current video to the end, not to leave halfway to watch another one. If you have so little confidence in your videos that you want to encourage people to leave halfway, then you should not be making videos altogether.
Give the video an honest descriptive title. It is a crime to make viewers sit out minutes of rambling, only to discover that the subject from the title is not even mentioned in the video. Yes, you can attract viewers with a misleading title; but if you think it ethical to draw their attention with deceit, you do not deserve the air that you breathe.
Use an image frame from the actual video as the "thumbnail" (which is the example-picture shown in the list of videos for the viewer to choose from). Using a custom image that does not even occur in the video is misleading and may cause disappointment in the viewer. It would perhaps be best if custom example-pictures were not possible altogether.
Only publish a video when you have something substantial to say or show. Do not make hollow, obligatory videos without substance, like daily or weekly talks when you really have nothing to say, or reviews or discussions of new products or current events that you know little of but are just using to attract viewers by riding along on the "hype".
This concludes the advice. If you follow these instructions to the letter, you will have a more than reasonable chance of forever remaining unsuccessful.