To literally answer your question, search for "Edit the system environmental variables", also found via Control Panel --> System --> Advanced System Settings. Then, click "Environmental variables...".
Under System variables, find "Path" and add the absolute path of the folders you want (separated by semi-colons).
That being said, I have a feeling your real issue is, "I can't build SFML correctly". Is that so?
Have you looked at the "Getting Started" tutorials? https://www.sfml-dev.org/tutorials/2.5/