1) Open Metamask, and select "Custom RPC" from the Network Dropdown.
2) In the "Custom RPC" Settings, add in the xDai network details and click Save:
Network Name: xDai
New RPC URL: https://rpc.xdaichain.com/
Chain ID: 0x64
Block Explorer URL: https://blockscout.com/xdai/mainnet
When bridging tokens from Ethereum, BSC or elsewhere you may need to add the custom token to MetaMask to view it. The address on xDai will be different from the address on Ethereum or BSC.
The easiest way is to click on the fox icon in BlockScout or OmniBridge for the token you are adding, then complete the process through the MetaMask popup.
If you want to add manually in MetaMask, go to Assets -> Add Token, Paste in the address (symbol and decimals should populate if you are connected to xDai) and save.
Instructions for using MetaMask with a Ledger or Trezor: https://metamask.zendesk.com/hc/en-us/articles/360020394612-How-to-connect-a-Trezor-or-Ledger-Hardware-Wallet
This troubleshooting guide from 1Hive is helpful if you are experiencing issues with MetaMask and xDai.