Step 1: Testnet Zip File

Download the latest testnet zip file from https://assets.nu.fi/extension/testnet/nufi-cwe-testnet-latest.zip 

Step 2: Chrome Beta

Download Google Chrome Beta

Step 3: Developer Mode

Open chrome://extensions/ in the Chrome Beta browser and allow developer mode.

Step 4: Installation

Drag and drop the downloaded testnet zip file into chrome://extensions/ window, where it will be automatically installed.