When you click on the job description from the open job listing, there is a green Apply button at the top right corner. When you click on that button, it will take you to a login page for governmentjobs.com.
If you don’t have an account, you will have to create one. If you have a login, use your credentials to login. After you’ve logged in, you will be required to complete the application for the job for which you’re applying.