You have your high-level story set out, and are now ready to prepare your slides. Below you find some general advice.
Here you find an example presentation of myself. It probably violates some of the advice below, but just as an illustration.
Intended for: BSc, MSc, PhD
Never put up your whole slide at once. Enter items one-by-one. Guide your audience where they need to pay attention.
This is probably the most important advise of all. Never ever put up a complete complicated slide at once, with a lot of text, figures, etc. Your audience will be distracted, does not know where to look, and definitely stop listening to you. Instead, make every item/figure/arrow etc. pop into the screen one by one, at the correct moment when you want to talk about them (bring a clicker!).
(I usually start every new slide completely blank, with only the title, to make a transition/bridge from the previous slide. I then gradually pop in new elements into the slide, for example item by item. The easiest way to achieve this is to first construct your entire final slide, and then repeatedly copy-paste it above and remove the last element you would talk about. Repeat this process until you get back to the empty slide with only a title. Clicking through the slides in a forward direction should give you a nice slide where items/figures/arrows/boxes enter one by one.)
Use arrows, colors, bold (don't use excess illustratory figures that distract).
You can perfectly guide your audience through a slide by using arrows, bold letters, colors, etc. Do you want to emphasize a certain word in a schematic figure: pop in a red box around it! Do you want to explain a certain symbol in an equation: put an arrow towards it, and write the mean of the symbol at the end of the arrow. Make absolutely clear to your audience what you are talking about at this specific moment, and where you want their attention.
Don't put too much text on a single slide.
First of all, your audience will start reading, and stop listening to you. Second, the message of the text won't even get across either, since they are distracted by you talking. Third, it's just boring, and half your audience will zone out.
Actually, prefer - conceptual - figures over text in general.
Actually, you don't want to use too much text anyway. Text is a great way of communication, but a lot of information is easier to process in a visual way (this side is not a great example of it unfortunately). When you can, add conceptual illustration and figures instead of text: it will help your story a lot.
Don't use too many equations.
In general, you want to use as little equations as possible in your presentation as possible. People in the audience that are not familiar with the specific notation will get lost very soon: they don't know what the symbols mean anymore, and therefore have no clue what you are doing. Try to explain what happens in a more conceptual way. There are two exeptions to this rule: 1) you talk to a super expert audience, of which you are sure they recognize all your notation conventions, or 2) you teach a lecture, because the lecture slides are teaching material, and therefore the students need to be able to study from them.
When you do use equations (some are often necessary), make sure you introduce every symbol.
First pop in the equation into your slide. Then, one-by-one add arrows to the specific symbols in your equation, and put a textual explanation at the end of the arrow. When you walk through the equation this way, usually you automatically explain it in words. For example: "We define the value of a specific state as" (V(s)=) "the expectation/average over all possible future traces" (E[.......]) "of the cumulative reward of each of these traces" (Σt rt).
Add questions to your slides.
This especially applies to Lecture slides, but you may also do it once or twice in a research talk. I will get back to the importance of audience engagement in the Presenting section, but make sure that at certain points in your talk you activate the audience through a question. Pop the question into your slide to make sure you remember. Conceptual questions to make sure the audience reflects/understands are best.
Time your talk at least once.
You are almost ready now, but make sure to time your presentation at least once. A few minutes more or less then the required time is totally fine, but you don't want to enter a 15 minute talk with 4 or 40 minutes of slides.
Design/export your slides in a platform independent way.
The best thing is to always bring your own laptop to a presentation, so you can connect to the beamer yourself. This is possible 99% of times, and ensures your on-screen slides will look the same as they look at your laptop. In addition, you can use your own pointer. However, if you are not sure this is possible, then you may have to use the local computer. In that case, the best option is to have your slides ready as a pdf. Another option is to use Google Slides, which you can bring up online (but this does require an internet connection at the presentation spot).