Open MetaMask and click the list of networks at the top, then select "Add Network".
You will be prompted for additional information. Use the table below to complete the step. Below you will see multiple options for RPC URL and Chain ID corresponding to mainnet vs testnet, and the various shards within each.
The example below shows the configuration that needs to be done to connect to Harmony Mainnet on Shard 0:
Click the Save button and your configuration should be done!