Meditation. It doesn’t happen all at once, but if you keep at it, you can explore the deepest depths of your psyche as often and as long as you want.
The main thing you’re going to want is a map. The maps are legion. A popular map you can use is chakras. Another one is OBE. People tend to get hung up on the specifics of the varied paths. They all in fact do the same thing. Create patterns in the subconscious through belief so that you’re not just fumbling around in chaos.
Whatever map you use, take it fairly seriously and suspend disbelief. Sit down, close your eyes, refrain from sudden or sharp movements, and visualize what they want you to visualize, try to look for the results. Try to relate the results back to the map. Do it once a day. Switch maps occasionally.
Eventually you’ll get to the point where you can make your own map. Can take years to get there. But even if you keep using other people’s maps, then you’ll be able to explore some strange and interesting places.
All of these interesting places are part of your subconscious. Even if you do OBE and are exploring otherworldly realms and you fully believe that, the experience is still getting filtered and guided by your subconscious. It has to hit your body and mind somehow.
It’s a great deal of fun, but look out for the Dark Night. It happens almost immediately after you start to really believe in what you’re doing. It’ll hit you like a ton of bricks and you should be careful not to take it too too seriously. Subconscious stuff just starts bubbling up, and it even affects your waking life too.
Your best defense is to just stay grounded, go back to the basics, and just push through. Good luck.