How do I choose the sidebar I want to use for a page with sidebar template?

After you create the sidebar at Appearance > Sidebars activate it by:

Step 1: Go to Appearance > Widgets and populate your sidebar with widgets.

Step 2: When editing a page go to the Page Attributes menu on the right and choose a template with sidebar from the Template select box (Page with Left Sidebar or Page with Right Sidebar).

Step 3: Go to the Sidebar box and select the sidebar you want to display for this page.