Design
Pages created in Sitefinity are based on page templates. These pages inherit their parent template's layout & design. By working with page templates, web site administrators can establish a consistent layout & visual experience for the entire web site. Likewise, editors can inherit these designs and focus purely on the content being created.
Important concepts before starting
Create a new ASP.NET Master Page
Follow these steps to create a new Sitefinity Page Template:
Create a new ASP.NET Theme

*
{
padding: 0; margin: 0;
}
body
{
background-color: #EEE;
font-size: 18px;
}
p
{
margin: 1em 0 1em 0;
}
.wrapper
{
width: 700px;
background-color: #FFF;
border: 1px solid #999;
margin:2em auto;
}
.header
{
padding: 0.75em;
border-bottom: 1px solid #999;
}
.header h1
{
font-size: 2em;
}
.sidebar
{
width: 160px;
float: right;
padding: 0.75em;
}
.main
{
padding: 0.75em;
margin-right: 190px;
border-right: 1px solid #999;
}
.footer
{
padding: 0.75em;
border-top: 1px solid #999;
}
Add the ASP.NET Theme to Sitefinity
1. Log into Sitefinity and click Administration -> Settings.

2. Switch into Advanced settings

3. Select Appearance -> Frontend Themes and click the Create New button.

4. Type the Name and Path to the new ASP.NET Theme.

Add the Master Page Template to Sitefinity
1. Log into Sitefinity and click Design -> Page Templates

2. Click the Create a template button.

3. Type the name for this new Page Template, select Use Template and then click Select another Template.

4. Click the Use your own .master file button.

5. Select that master page from the list of files and click the Use Selected button.

6. Click the Create and go to add content button.

7. Click the Theme button and select the new theme from the dropdown.

Populate the template with content
All content added to this template will be automatically inherited by any page based on this template. Consequently, it's possible to place a page title, footer, navigation, sidebar or any other element onto the template and these elements will appear site-wide.
1. Toggle into Content mode and drag & drop a Content block widget onto the page.

2. Click Edit to modify the Content Block.
3. Drag & drop the Navigation widget onto the template, click Edit to modify.

4. Click the Publish button to save this template.
Create a page based on this template
1. Click Pages and click the Create a page button.

2. Click the Select another Template button.

3. Select the custom template and click the Done button.

4. Click the Create and go to add content button.

Next steps...
Populate the page with content and create more pages.
Content can now be dragged & dropped onto the page. In additional, multiple other pages can also be created. The entire web site can be built and populated. All of these pages will be based on their parent template. As a result, all page will share a common look and any changes to the underlying template will be spread to all of its child pages.
For more information, check out the Sitefinity Designers Guide.