Enable Anonymous Access at Site Collection in SharePoint 2010

To Enable Anonymous Access for a Site Collection, follow below steps

Step 1: Navigate to Site settings and select Site permissions under Users and Permissions

Step 2: Select Anonymous access from ribbon. 

Step 3: From the popup, select the suitable option and click Ok. Now the anonymous access is enabled at Site collection.

Read this article to know how to Enable Anonymous access at List/Library Level.