There are two parts to this: Background Images and Template Pages. Let's look at them separately.
*** Background Images
A background image is a single PDF page from a PDF file, or an image like a JPEG photo file, that can be displayed as the background of any sheet of any page. It is like the lined backgrounds in that respect and it is chosen from a list. The one image or single PDF page is shared by all sheets that have it as a background. So, if you assign background image "Time Sheet" to three sheets of any pages, if you change the image or PDF page associated with that background (using the Background Image List's "Change" button) all three sheets will look different when you turn to them. This is good and bad -- it saves memory by having only one copy and lets you update lots of pages at once, but it means that you could inadvertently change the look of many pages by mistake. Also, if you have multiple pages in the PDF file you need one background image for each.
Annotation pages have a copy of the entire PDF file associated with it. If you create two annotation pages from the same PDF file, you will have made two separate copies of the original PDF file.
To create a background image, you must get the file (PDF or JPEG) into the File-Import folder. You can do this by using iTunes and adding the file to the File Sharing for Note Taker HD, or you can do "Open In..." of that file from some other app (like Mail) and then indicate that you want the file to be added to the File-Import folder by pushing the "Add to Folder" button.
Once the file is in the File-Import folder, you can then go to the Background Images (from the App Tools) and then create a new background image, give it a name, and then "change" it to have that file, or single page of a multi-page PDF, as the "image" for that background.
Let us say that you used "Open In..." with a file was named "day-planner.pdf". After pressing the "Add to Folder" button, the app will now indicate "Copied File" and switch to displaying the Background Image List.
Once in the Background Image List, press "Add New Item" and you'll see "(Unnamed background)" added to the list. Tap the text field in the upper right (above the word "width") and type the name you want for the background image (e.g., "Day Planner") and then press the Done button ON THE KEYBOARD (this is important).
Now, press the Change button. Select the "day-planner.pdf" file from the list of the contents of the File-Import folder on the left. Make sure the page you want as the background is selected and showing (a background can only be from one page of even a multi-page PDF file). (There is a list of page numbers.) Press the "Save" button. Leave the "Output as PDF: YES" and the "See PDF" as they were by default. Press the Done button in the upper left to return to the List of Pages.
You then can set the background of any sheet of any page to have that background, just like you would with narrow vs. wide lines.
For example, create a default blank page. Use the Page Settings (the Gear in the upper right) to set the "Background" setting. Select, continuing with the example, "Day Planner" in the second set of backgrounds listed. Press Done to get out of the Page Settings. Set up anything else you want about the page, such as the size of the Detail Area, the pen color and width, etc. Give the page a name (like "Day Planner Template") and then press the Done in the upper left to get back to the List of Pages.
That's a lot to do. Also, you really might not want to do this. Instead, you might just have a single or multi-page PDF file that you want to use as the background of a single or multi-sheet page and not use that mechanism. You can make an "annotation page" that has its own copy of the PDF file associated with it and has each sheet associated with different pages of that PDF file. You create an annotation page by either doing an "Open In..." of the PDF file and pushing the button on the window that comes up that creates the annotation page. Alternatively, you can put the PDF file in the File-Import folder and then use the "+" and "Annotate PDF File" on the List of Pages to bring up the contents of the File-Import folder and select a PDF file to be annotated. You can use the same file any number of times -- it will be copied for each use.
*** Background Images vs. Annotation Pages
If your goal is to have a background page (e.g., the Day Planner page) and each day create a new, blank one to use, I think it's better in this case to use a background image rather than repeatedly making an annotation page. If you want to update the background with something totally different, just make a new Background Image with a different name, leaving the old ones alone.
*** Template Pages
Now, once you have a page that looks the way you want, instead of creating it each time, you can have a "template page" that is duplicated to make the new page.
A template page is just like any other page, except that it is also in the template page list. This list is shown when you press the "+". That's easier than finding the page in the List of Pages and then calling up the Duplicate command, but it is the same.
To get to the list of template pages in version 6.0, go to the Templates folder. The "doc order" of the docs there is the order on the "+" list, #1 being first, #2 second, etc. (Not the "modified" order.) You can copy docs into the Templates folder using the "To Folder" tool.
In version 5.1, in the List of Pages, press the Tools button. Select "Template Pages." It will say "Reorder Template Pages" at the top. Press the "Select Pages" button at the bottom. Find, for example, your "Day Planner Template" page in the list and tap it to select it. The box next to it should be checked. Tap "Reorder" to make sure it's now in the list. (You can reorder the list at that point if you have more than one template page.) Tap outside of the popover to dismiss the Template Page list.
Tap the "+" button in the upper right. The "Day Planner Template" should now be in the list of pages you can create. Tap it each time you want a new page.