Open Google Chrome
Go to the Chrome Web Store
Search for the extension you want
Click the extension name
Click Add to Chrome
Confirm by clicking Add extension
Wait for the extension to install
Open the extension from the toolbar or Extensions menu
If you want to download the extension file manually, get the CRX file from a trusted source
Save the CRX file to your computer
Open Chrome and go to chrome://extensions
Turn on Developer mode
Drag and drop the CRX file into the Extensions page
Confirm the installation if prompted