Before you are able to add ETH for bonding, you must first authorize all tBTC operator contracts. Refer to this section for a refresher on how to authorize operator contracts:
Once you have authorized the operator contracts, you will be ready to add ETH for bonding.
In the Add ETH for Bonding table below the Authorize Contracts table, you can see the total amount you have available for bonding. Before you have added any ETH for bonding, this amount will be 0 ETH.
Bonded ETH is ETH that has been bonded. Available ETH is ETH that is available to be bonded upon selection for a signing group. You will need to have some amount of ETH bonded in order to be selected to operate.
Click the "Add ETH" button on the far right of the table row. In the modal window that appears, enter the amount of ETH that you would like to add and submit. You'll now see in the table that it reflects the amount of ETH that you just added. You can repeat these same steps to add more ETH.