Write basic App Script GET function for Google Sheets

I need a basic Google Sheet / App Script function that will check a Sheet, and for each row generate a GET request then parse the response.

- Check "Input" sheet for any row where 'isProcessed' column is empty
- Generate GET request, parse 'username' column value in URL of request
- Parse JSON response and append onto "Output" sheet
- Wait 1 second
- Fetch next row..