That's a pretty broad question, but the basic answer is that it's not as difficult as you may think. There are recognized traditions for calling for a 'truce', or offering a surrender - showing a white flag is an example of this.

In any case, it's not difficult to send a small party to where the enemy is encamped. If you approach openly, in small numbers, with no obvious hostile intent then you generally wouldn't be killed out of hand and they'd wait until you were within hailing distance to call out and have a conversation - offer terms of surrender, ask for a truce, whatever it is. Over time a lot of norms and informal codes of conduct have developed in terms of how emissaries were to be treated - and this led to infamy whenever these emissaries were abused. The term 'shoot the messenger' probably applies here.

In more modern eras, negotiations between nation-states with no formal diplomatic relations would often be carried out through third parties and neutral nations. For example, towards the end of World War 2 Heinrich Himmler made inquiries about a negotiated peace through contacts in Sweden, which was a neutral country. In more recent times, the US Government often uses Swiss or Swedish embassies as a way to get in contact with countries like Iran or North Korea, for example when they want to get detained Americans out of those countries.

Long story short, it's really not that difficult to get a message to a hostile party - either by sending it with someone who the hostile party won't kill outright, or by approaching in a obviously non-threatening way. Most of the time it's worked out, and the position of diplomat/emissary is a respected and protected one.