how do i backup i pad air ?

It is a good idea to back it up to either iCloud or iTunes. Back up and restore your iPhone, iPad, or iPod touch using iCloud or iTunes - Apple Support

I can't tell from your original question what is causing the message; you may want to reply with the exact wording of the message.

Where are you getting this message. Is it something to the effect that your iPad hasn't been backed with iCloud for XX weeks. Backups occur..... Something like that? If so, then you must have selected to backup with iCloud but your iPad hasn't been plugged in to recharge and locked in order for an automatic iCloud backup to take place.

Try Settings>iCloud>Backup>On>Enter your iCloud password when prompted to do so then tap on Backup now.

Read the link that FoxFifth posted for you above. That explains how to backup with iCloud or with iTunes. Before she backups up either way or does anything with regard to iTunes. It is very important that she provide more information as to what the exact wording of the message is that she is getting. If she has never used iTunes before, it will take some work for her to make her computer's iTunes work properly with her iPad without erasing it.

The quote below from Create and delete iPhone, iPad, and iPod touch backups in iTunes - Apple Support shows the iTunes backup location for a Mac and various Windows PC operating systems. iTunes doesn't have an option to change that to an external drive although there are ways in Windows (that I don't currently have details or ability to clearly explain) that may allow you to get around that.

Where your backups are stored

The folder where your backup data is stored depends on your computer's operating system. Make sure the backup folder is included in your data-backup routine. iTunes places the backup files in these places:

• Mac: ~/Library/Application Support/MobileSync/Backup/
  The "~" represents your Home folder. If you don't see Library in your Home folder, hold Option and click the Go menu.
• Windows Vista, Windows 7, and Windows 8: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\To quickly access the AppData folder, click Start. In the search bar, type %appdata%, then press Return.
• Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\To quickly access the Application Data folder, click Start, then choose Run. In the search bar, type %appdata%, then click OK.

iTunes itself won't let you change where device backups go, it will use your computer's home drive. But you could try using junction to redirect it to your external drive (the drive will need to use the same drive letter whenever you connect it to backup to it) : Re: Can't Change iPhone Backup Location

You can backup to the main drive on the computer and then drag-and-drop (or cut-and-paste) from there to the external drive - using the 'junction' should mean that you don't have to do any extra steps.

Not everything is stored in a backup, neither iTunes nor iCloud - for iTunes purchases (music, films, apps et) an iTunes restore will look to pick them up from the iTunes library (so you should also do File > Devices > Transfer Purchases to copy any iTunes purchase over to the iTunes library) ; an iCloud restore will redownload what it can from the store (so will rely on the items still being in the store, not having moved countries since buying them, and the type of content being redownloadable in your country ; audiobooks and tones are one-time only downloads in all countries, so won't be restored).

How long an iCloud backup will take will depend upon how much is her content e.g. photos, documents, files ; iTunes purchases aren't copied back up to the cloud, they will be available to the restore (if they are remain in the store) - and how quick your connection is.

Backing up to iTunes or iCloud : Should I use iTunes or iCloud to back up my iPhone, iPad, or iPod touch? - Apple Support

iCloud backup overview : iCloud: iCloud storage and backup overview

iTunes backups : Create and delete iPhone, iPad, and iPod touch backups in iTunes - Apple Support

jimtownrc wrote:

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If she gets set up with I Cloud how many hours does it take to back up, i'm not sure if she has 6 or 8 g on the Air.

What are the obstacles of getting it dome right, i see lots of users with all kinds of issues?

In my case a backup to iCloud takes only a few minutes. I have a high speed internet connection and not a lot of data.

There shouldn't be any obstacles. The only users who typically post here are the ones who have had an issue. The many millions who haven't had an issue have no reason to post here.

To backup to a computer she'd need to use iTunes.

She can backup to the cloud instead (or as well as to iTunes), and that has some advantages - if she has a problem with the iPad and needs to restore to a backup then she could do so on any accessible wifi network ; only backing up to a computer's iTunes could be an issue if she was away from home (e.g. holiday), she would have to wait until she could reconnect it to her computer before she could restore her content.

The advantages of using a computer, or at least copying purchases over to it, is that you have your own copy of any purchases so aren't reliant on them remaining in the store. Most items do stay in the store, but rights-holder occasionally remove media (music, films, tv programmes ; though if she hasn't bought any that won't be an issue)