2. Open System Preferences and click on Time Machine
3. Click "Select Backup Disk"
4. Choose your external hard drive from the list
You can also check the box to show Time Machine in the menu bar. This will give you a visual indication of when the backup is done. Be sure to plug in your backup disk often so you always have a current backup. When you receive your new Macbook Air your backup files will be used to migrate your data.
Google Drive Method
This method is less comprehensive and you will have to manually drag all content you would like to backup and transfer to your new computer, however it is a 100% free option. Items you might want to consider backing up would be located in: Desktop, Downloads, Documents, Photos, Music, and Movies.
The easiest way to copy your files to Google Drive will be to use the Google Drive client on your mac. It will be located in the Applications folder.
1. Login to google drive on the web via your browser and click settings.
2. Make sure the convert uploads to google docs editor format is unchecked.
3. Click new, select folder and name it Backup.
4. Open Google Drive from the applications folder and then drag and drop the folders you want to backup
When you make a change to a file that has previously been backed up to your Google Drive you will need to re copy it over to make sure the most up to date version is copied to your new MacBook Air.