As the help author you don't need to do anything special to create normal context-sensitive topics. They are simply normal TOC topics and normal popup topics. They become "context-sensitive" when they are called directly from the application. What makes field-level popup topics a little different is the fact that they are called on their own, without the help viewer, but when the same popup topics are called within the help they behave the same as other popup topics. The difference is in the way the topics are called, not in the way they are created.
For more background information see the Context-Sensitive Help & Popups chapter in the Reference section.
Normal context-sensitive topics are simply normal topics in the TOC. Just follow the instructions in Creating new topics to create a new topic in the TOC.
These topics become context-sensitive topics when they are called directly from the application. How this is done depends on the programming language and your output format. For details see Application calls to context-sensitive topics.
Field-level popup topics are simply normal popup topics. Just follow the instructions in Creating popup topics to create and edit popup topics.
Requirements for field-level popups:
Add context numbers: Plain text popup topics used in HTML Help must have help context numbers! This is required by the Microsoft HTML Help API for popups and if your popup topics do not have help context numbers they will not be exported to the internal popup text file in the CHM.
No formatting or graphics: Plain text popups are really plain text only. Any formatting, graphics, tables etc. will be stripped when they are displayed as field-level popups. Take care to make your popup content comprehensible as plain text.
Changing the name of the internal field-level popups file in the CHM:
By default, the internal field-level popup file is called CSHelp.txt. You can change this in Project Explorer > Configuration > Publishing Options > HTML Help > Extended .HHP Settings. See Extended .HHP Settings for details.
Using field-level popups:
To use field-level popups your programmers must call the popup topic inside the help file from your application, using the appropriate API calls for HTML Help. The syntax of these calls depends on your programming language. You can download tutorials for most current languages from our website. See Application calls to context-sensitive topics for more details.
Auto-generating your popup topics from a map file:
If your programmers provide you with a "map file" of the topic IDs and context numbers for the elements in the program that need to be documented you can also generate your field-level popup topics automatically! See Auto-generating context-sensitive topics for details.