To add a sheet:
Bring your dashboard into edit mode by clicking on the pencil in the top right corner > Click Add a Chart > Choose Sheet > Add at least one row and one column > click on the blue "+" in any cell and add a metric.
Repeat this process for as many cells as you'd like. Once you've built the sheet with your desired metrics you can easily adjust the date range by clicking the orange pencil in the top-right corner of the Sheet and adjusting the Sheet options.
When you're finished, click the green check mark in the top right corner to view the Sheet in your dashboard.