If you want to interact with this bridge, first of all you should have are installed wallet extensions (on this bridge available: MetaMask, Trust, WalletConnect, CoinbaseWallet or imToken)
You can also find official xDai bridge FAQ on xDai documentation page.
1. You need to have wallet extensions installed for each network you plan to interact with. On this bridge available wallets such as: MetaMask, Trust, WalletConnect, CoinbaseWallet and imToken.
2. You need to have some amount in the native network token to pay the network fees.
3. Connect wallets to the bridge site.
4. Make sure this bridge supports the token you want to transfer and only then make it.