Simply drag and drop a folder to Prepros window to add new project.
By default Prepros watches all files in project folder. This is fine for small projects but if you have large projects right click and filter out any folder that doesn't need to be compiled or refreshed automatically. Files and folder that are not watched for change have icon in Prepros window.
CMD+L depending on your platform or use the earth icon on the toolbar to open live preview.
Prepros watches your files and compiles them whenever you save them in code editor. If you want to manually compile them just click the file in Prepros window then click Process.