[development] How do I send a file to the user?

Hi I want to do the following: The users fills out a form and depending on that data an excel export is generated. The user should be prompted to store/open the file. The code to create the excel data is working fine. But I don't know how to do the file sending stuff? Is hook_file_download the right way to go? If yes, after I have the excel file content stored in a variable how does it get to the implementation of hook_file_download? TIA Ernst