Click on your name in the top right and select "Account Settings"
Select Job > Recruitment Workflows and then click on the workflow you want to edit
Or if you are within a job already you can just click the cog wheel here
Click the plus button where you'd like to add your new stage
Select "From Scratch" to create a new stage or "From Library" if you want to use an existing stage
Give your stage a name and then select a milestone (for reporting purposes)
Your stage is now created. Click "Save & Close" to return to the workflow or you can continue to add an email template by clicking the new template button