Just create a normal PlatformIO project with the microcontrollers your library is targeting, then start developing the library in a folder in lib/ with testing code in the src/ folder of the project. If development is done, publish the lib/ in a repo standalone (or pull it in via a git submodule in the first place).
The needed folder layout of the library is always written in the README.
If you strictly want to write an Arduino library, you should add a library.properties of the documented format to the library.
An example is e.g. GitHub - adafruit/Adafruit-MCP23017-Arduino-Library: Arduino Library for Adafruit MCP23017