how to install new background in Pages

Pages '09 or Pages v5? The explanation is different for each.

Here are the steps to get a pattern onto the pages of your document.

1. From the View menu, choose Show Layout
2. From the Toolbar, click Shape and choose a rectangle.

   With the shape still selected, click the Arrange tab on the side Format panel. Set Text Wrap to None.

3. Now stretch your rectangle to cover the document body outline. Use the yellow guides.
4. Again, on the Format panel, click the Style tab.
   1. Click on the word Fill to expand this section.
   2. Select Image Fill
   3. Select Tile, or scale to fill depending on your background size
   4. Click Choose, locate your background, and open it. You should now have a background in the rectangle.
   5. Use the opacity slider at the bottom of the Style tab within the Format panel to adjust for text legibility.
5. From the Pages Arrange menu, choose Section Masters ▸ Move Object to Section Master.

   Your background will now appear on all document pages, and everything else will appear above it.

6. From the View menu, select Hide Layout.

Here is an example. I use patterns from here.

The subtle site has a plugin for Photoshop that makes the patterns available in that application. The @2 versions are for retina displays. They could also be used for web page background patterns.

When you move the selected background to section master, it becomes the section master object. When you make section master editable, yellow grips will appear on the background rectangular shape. Unless you need to edit/change your background, you would simply leave the section master alone.