Summarize this article:

Plug one end of your iPhone's or iPod's charging cable into the charging port at the bottom of the iPhone or iPod, then plug the USB end of the charging cable into your computer. Its app icon resembles a musical note on a white background. If iTunes prompts you to update it, click Download iTunes on the prompt and then restart your computer after the update completes. It's an iPhone-shaped icon in the upper-left side of the iTunes window. Doing so will open your iPhone's or iPod's page. It may take this icon a few seconds to appear. If you don't see this icon appear after a minute or so, try unplugging and re-plugging in the charger cord, or try using a different USB port on your computer. It's on the left side of the page. Doing so will open a list of apps that have files associated with them (e.g., Pages). Click an app to which you want to add files. In the section of the page that's on the right side of the iTunes window, you'll see a list of documents (if there are any) that are associated with the app. For example, if you select Microsoft Word, you'll see a list of Word documents stored on your iPhone or iPod. You can delete a file by selecting it and then tapping the Delete key. To save a file onto your computer, select it, scroll down, click Save to..., select a location on your computer, and click Select Folder. It's in the lower-right side of the iTunes window. Doing so will open a window. Click a file that you want to add to the app in question. You may first need to select the file's location folder on the left side of the window. It's in the bottom-right corner of the window. Doing so will place the file in the selected app's "Documents" section on your iPhone's or iPod's page. This button is in the lower-right side of the iTunes window. This will upload the file to your iPhone or iPod. You can click Done after the sync completes to return to the main page.

Summary:
Connect your iPhone or iPod to your computer. Open iTunes. Click the "Device" icon. Click the File Sharing tab. Select an app. Review the app's contents. Scroll down and click Add File. Select a file. Click Open. Click Sync.