Navigate to Extensions>>Module Manager.
(Note: all available modules show up under this Module Manager, even though some of them are built into Joomla! and are not extensions.)
On the left side of the page, on the third line, there is a menu that lets you select the position. You also can create a new position name on the fly but typing it in the same space as the menu. (In that case, make sure the same position name also appears in the template file.)
Modules that are not published will not display.
Modules can be assigned to unused positions (positions not available in the template) if you want to have them published but not displayed in a position (for example, if you want to display a module in content). See How do you put a module inside an article? for instructions on putting a module inside an article.
Multiple modules may be assigned to the same position. They will be displayed in the order shown for modules in that position in the module manager.
If you want to display a module in more than one position, use the module manager new icon to create an additional copy and assign that copy to the second position.