Where to put my header file

Development Tools IDE 1.x
2

Put it here:

<Sketchbook location>
|_ libraries
   |_ f1
      |_ f1.h

(where <Sketchbook location> is the path shown in the Arduino IDE at File > Preferences > Sketchbook location)

Notes:

The f1 folder can be named anything you like.

The Arduino build system does use the match between the header file name (f1.h in this case) and the folder name (f1 in this example) to determine while library is used when multiple libraries contain a file matching the #include directive, so it is best practices for the folder name to match the header file name. But in most cases there is only one library that matches, or if there are multiple the Arduino picks the right one, so it's not something to worry about too much if some other folder name seems more appropriate to you.

There is an alternative "recursive layout" for libraries:

<Sketchbook location>
|_ libraries
   |_ f1
      |_ library.properties
      |_ src
         |_ f1.h

You can learn about it here:
https://arduino.github.io/arduino-cli/latest/library-specification