If you have not downloaded MetaMask please download from here
Click on the custom RPC to install Coston Test Network
Here is where you need to fill in the following information from the Flare Network documentation
When complete it should be filled in and look like this *note you will not have URL is already present, I have the network already installed.
You will now see Coston Test Network show up in your list of available networks
Lastly you will need to claim your CFLR from the Faucet. This is the test net currency. You can claim from this website https://faucet.towolabs.com/ you need to copy and paste your address from your MetaMask. Once you claim you should have CFLR in your wallet and ready to go!