1. Download El Capitan installation from the App store. It might complain that you already have it installed but download it anyway. It will be downloaded to /Applications and be named Install OS X El Capitan.app
2. Create a bootable image from the El Capitan installer on a USB stick larger than 6GB. In a terminal run:

1. Create Bootable Usb El Capitan

For an OS X El Capitan installation, the file gets saved as a disk image named 'InstallMacOSX.dmg'. To install, open the disk image and run the installer named 'InstallMacOSX.pkg'. Nov 07, 2015 Install El Capitan. Now that the installation USB drive is ready, let's install OS X El Capitan! Boot from the USB Drive. Restart your computer and boot from the USB drive (Press Esc/F8/Del to access the boot selection menu). Use the arrow keys to select Boot OS X Install from Install OS X El Capitan (It should be.

where MyVolume is the name of your USB stick. Change this to match the name of your USB stick.

3. Physically install SSD into the target laptop.
4. Insert the El Capitan USB stick into the target laptop and power up. (This might take a while.)
5. When the menu finally turns up, choose Disk Utility and format the SSD as Mac OS Extended (Journaled) with a unique name.
6. Once complete, exit from Disk Utility and return the main menu.
7. Choose to Install OS X on the formatted SSD.
8. Follow the prompts until El Capitan is installed.

Here’s link to a video on how to physically install the SSD as well as run the software installation.

Create Bootable Usb El Capitan

If you’d rather install Yosemite on the SSD then follow the How to Install Yosemite on a New SSD guide.