Typically they are accessed the same way as any other area.

Some, however, require a little more attention to detail. For example, there may be a phrase you have to say to get beyond a certain point, but the phrase will be in the area.

Often, if an area has a hidden entrance (one that you can walk through but doesn't show up as an exit), it will be an explore one.

Many can be explored solo, but generally you are better going with someone else first time (so they warn you about the dangerous bits).

Personally, I rank in a number of explore areas, so really just grouping with a character like mine for ranking will expose you to a number of them.

There are no requirements to reach explore areas today, as far as I'm aware.

Area explores may well have "tricks" to them that make them considerably easier. For example, there's one where you can disguise yourself and that means that many of the mobs won't be aggressive to you, whereas if you are not disguised, they will.