There is a single command for creating new items, whether the new item is conceptually `flat' (those without subitems) or `deep' (those with subitems). For example, a new paragraph might be considered flat and a new section deep.
At the simplest level, you select the preceeding item
which is of the same type as the new item you wish to create. You then
type either meta-l meta-i (to place it after current
position) or meta-l meta-I (to place it before current
position).
This will create the empty template for the item and put it in the appropriate place. For example, if you wish to create a fourth paragraph, you simply click on the third and type meta-l meta-i (or click on the fifth and type meta-l meta-I).
For `flat' items, this is sufficient. For `deep' items it is slightly more complicated. If you wish to create a new subsection, then you have to indicate that you want a new `deep' item. You do this by selecting an item which is conceptually at the level of the subsection (the title, for example) and issuing the meta-l meta-i command.
Note that both variants on the command will create the item and put in a placeholder if necessary. See sections 5.3 and 5.4 for a description of placeholders.