The simple/stupid/obvious answer: Keep a todo list. It takes some practice to define todo list items that are achievable in a consistent fixed amount of time.

The simple/stupid/obvious answer: Keep a todo list. It takes some practice to define todo list items that are achievable in a consistent fixed amount of time. But it's a worthy practice. Being able to boil a big abstract project into small component parts is (IMO) one of the most important skills of being a developer.