new hard drive installed. How do I install the OSX?

From your backup.

What year was your A1278 MBP made?

If after 2010, you should be able to boot into Internet Recovery (IR) and install the original shipping OS.

However, as already mentioned, restoring from your backup from IR would be simpler.

About OS X Recovery - Apple Support

Note that if it was from 2010, you would have had to install the firmware update to allow it to use IR.

If you cannot use IR, then you will need to install from the install DVD that shipped with your Mac.

If you don't have any backup you can install the system via USB memory. To do this you need to download El Capitan from a Mac computer and make a USB bootable with this command:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

where /Volumes/Untitled is your USB memory.

You first have to format the drive using Disk Utility, from Recovery

First all new drives must be formatted using Disk Utility.app—this is found in your Applications/Utilities folder, or via Recovery http://support.apple.com/kb/HT4718

The important part of formatting your drive for OS X is a GUID Partition Table and the file system Extended Journaled.

You can read more:

http://www.macworld.com/article/2990837/storage/how-to-format-a-startup-drive-in -el-capitan.html

Mac OS X: About file system journaling - Apple Support

ref: How to create a boot clone