Select the "Add Sheet" option in your toolbar.

A modal will open up asking you to select the template based on which you want to create the new sheet. Select the required option.

Refer this article to understand more about templates,

http://help.infurnia.com/en/articles/8504622-how-to-choose-a-template-for-presentation-sheet

Once you specify the template, a new sheet will be added as per the same.

To remove a sheet, select the same to view it first.

Then choose the "Remove Active Sheet" option which is right next to the "Add Sheet" option.

A pop-up will appear asking for confirmation, which when you confirm, the sheet gets deleted.