Steps for generating google credential keys

go to https://console.developers.google.com/apis/
create a new project
click |+| Enable API
search for sheets api
click on Google Sheets API and enable it
go to Creadentials section from sidebar
click on small down arrow in Create Credentials button to open a drop down menu and choose Service account key option.
choose Service account and give some account name
choose type as Project > Editor
set key type to JSON and click create
save the key file to app directory
Create google spreadsheet and give editor access to client-email, which is found inside .json file.
Edit the config file accordingly.

installation