You have to understand that before starting mainnet, we need validator nodes to mint blocks, since after the network starts, new nodes can only be added by voting of current validators.
I know how it works, the question is still the same, how can someone became a validator node?
First you must give us a public address to be able to include it in the allowlist of the network, then you must install the node (instructions on github), and finally start the nodes at the same time.