At this point, you’ll want to look to the time line at the bottom of the editing interface. There is a red cursor at the start of the recording. Leave it in place to start the recording with your first slide.
At the point the interface should alter so the slide is being shown as the main screen and the video recording is displayed in the upper-left corner. You should also see the slide added to the timeline.
Scroll forward with the red cursor to find the point in the video where you want to add the next slides. Stop the cursor where the slide should be inserted and repeat the process of adding the next slide in sequence using the plus button. In the end, your timeline should show all the slides, though in a longer video there will likely be less spacing between slides.