How do I add a custom network to MetaMask?

Could you please elaborate on the process of adding a custom network to MetaMask? I'm a bit new to this and would appreciate a step-by-step guide. Specifically, I'm interested in understanding how to input the necessary details like the network name, RPC URL, chain ID, and so on. Are there any special considerations or common issues I should be aware of when attempting this? I've heard of network congestion being an issue with certain networks, so would this apply to adding custom networks as well? Additionally, how can I ensure the safety and security of my transactions on these custom networks?