xDAI have created some documentation for adding the xDAI network to Metamask here: https://www.xdaichain.com/for-users/wallets/metamask/metamask-setup
Yearn have created the ability to add xDai to Metamask with one click, after connecting your wallet: https://chainlist.org/
Gimlu (a member of the community team) has also created this tool that allows you to add xDai to metamask with a with a single click: https://xdai-app.herokuapp.com/faucet
Once you have set up Metamask for xDAI, you will need to bridge some DAI to xDAI for gas fees.