Blog layouts are created through post categories. You’ll have to create new category in Dashboard > Posts > Categories or edit an existing category.
Find ‘Blog Layout’ drop down selection and select the desired layout (e.g. Right Sidebar). Below ‘Blog Layout’ you’ll find sidebar options. This is where you can select one of existing sidebars, if selected blog layout uses sidebar of course.
After that just click on ‘Add New Category’ button, or if you are editing click ‘Update’ button.
If you want to add created category to the menu, please look at Setting Up Menus article.