- 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
- Create a bootable image from the El Capitan installer on a USB stick larger than 6GB. In a terminal run:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app
where MyVolume is the name of your USB stick. Change this to match the name of your USB stick.
- Physically install SSD into the target laptop.
- Insert the El Capitan USB stick into the target laptop and power up. (This might take a while.)
- When the menu finally turns up, choose Disk Utility and format the SSD as Mac OS Extended (Journaled) with a unique name.
- Once complete, exit from Disk Utility and return the main menu.
- Choose to Install OS X on the formatted SSD.
- Follow the prompts until El Capitan is installed.
If you’d rather install Yosemite on the SSD then follow the How to Install Yosemite on a New SSD guide.