SPFX - Inject CSS into Modern Site

Hi Community,

Hoping someone can help.

I wish to create a SPFX app that will inject CSS code into a modern SharePoint site. I would like to use the most up to date environments/libraries in order to achieve this and was wandering if I could be give a step by step procedure, from setting up the environment, to building the app. I have tried following guides online, but keep running into issues whenever I try to build the application. Any advice would be greatly appreciated

Thanks,
AAMSP