It's easy to break down a task into components. In project mode, select the task to which you want to add some children, then do Edit->Outlining->Add Child (also available on command-}). Add any more children you need by just pressing return. This is a useful way of keeping track of your progress if you start on some task but are interrupted before finishing — just make a child action that describes the rest of the work to be done. You can nest tasks quite deeply if you wish.
The behavior of these constructs (called Action Groups in OmniFocus parlance) in context mode is possibly confusing at first look. The parent becomes available only after all of the children are marked complete, and so may not even appear until that point, depending on your view settings. It also appears after all of the children. The idea is that you move down the list completing the actions, and when they are all done, the parent is the next to be checked off, completing the group. Note however that if you are looking at a particular context, the parent may not appear if it is assigned to a different context, or has none set.