Start 2 nodes of a testnet on Windows (for non-Windows ommit the "start")
start monerod --testnet --no-igd --hide-my-port --testnet-data-dir .\node1 --p2p-bind-ip 127.0.0.1 --add-exclusive-node 127.0.0.1:38080
start monerod --testnet --testnet-p2p-bind-port 38080 --no-igd --hide-my-port --testnet-data-dir node2 --p2p-bind-ip 127.0.0.1 --add-exclusive-node 127.0.0.1:28080 --testnet-rpc-bind-port 38081 --rpc-bind-ip 0.0.0.0 --confirm-external-bind
Start cli wallet on same machine:
monero-wallet-cli --testnet --daemon-port 38081
Let it mine in one (or both) of the monerod windows:
start_mining <your_public_address_here> 1