First, you need to enable the module preview function, but clicking on Template Manager from the Extensions menu.
Then Click the Options button.
From the pop-up window that appears, Enable the Preview Module Positions option.
Now you have that enabled, view your website template in your browser.
The next step is dependent on the url of your page.
If you are viewing a standard page or article from your website, add ?tp=1 immediately after the URL of the page in your browser, and press return.
If you are viewing a page on your site that has generated using a variable (a search result for instance), add &tp=1 immediately after the URL of the page in your browser, and press return.
Each module position will know be overlayed on your website. To remove the overlay, either delete the command from theURL, or navigate to a different page.