When you have created your One pager, you need to start adding sections. You can add text sections and gallery sections.
In order to add a text section, click on + Add section and choose Text section.
Give the section a title and start adding content. This is a regular text editor, you can therefore add different block content (text, image, video, etc.). Choose background colour or image. When you are done, click on Back to section list in order to create more sections. The option Full height will determine the height of the section.