First, your node must be fully synced. Second, enable MetaMask connections from NiceNode <>> Settings > “Enable MetaMask Connections”. Third, in MetaMask > Add Network and enter “NiceNode localhost” with RPC URL “http://localhost:8545”. If your node is running and fully sycned, the Chain ID and other fields will be completed by MetaMask automatically and now click Save. For official info from MetaMask, follow MetaMask's local node guide