Some say that "The End." is the hardest advancement. Some say that "The Beginning." is the most difficult. But in my opinion, The advancement, "Adventuring Time" is by far the hardest. However, in this guide, I will try to help you all lighten up the load this advancement brings.

When should I begin?
You may begin at any time. The game will add any new biomes to the list, even before you have entered the End, which is also required for the advancement.

How do you get the advancement?
You must explore all 36 biomes in the Minecraft world.

What if I don't wanna?
Well, you can always do "/advancement grant <username> only minecraft:adventure/adventuring_time". However, if you want to do it legit, keep reading.

What biomes do I have to explore?
Practically every biome is necessary, except for a select few (Jungle M, flower forest, extreme hills, & swampland M). You have to set foot in these 36 biomes:

Forests-
Birch forest, birch forest hills, cold taiga, cold taiga hills, forest, forest hills, jungle, jungle edge, jungle hills, mega taiga, mega taiga hills, roofed forest, swamp land, taiga and taiga hills

Colder biomes-
Cold beach, frozen river, ice mountains and ice plains.

Warmer biomes-
Beach, desert, desert hills, mesa, mesa plateau, mesa plateau F, savanna, savanna plateau and stone beach.

Water biomes-
Deep ocean, ocean and river.

Other-
Extreme hills, extreme hills +, mushroom island, mushroomislandshore and plains.

That's too much. I'm out.
It may seem like alot, but it is not. Many of these biomes are easilly connected to one another, so if you find one, you may find two others right by it. I made the list smaller for you:

Beach, birch forest, cold beach, cold taiga, desert, extreme hills, forest, frozen river, ice mountains, ice plains, jungle, mega taiga, mesa, mushroom island, ocean, plains, roofed forest, savanna, stone beach, swamp land and taiga.

That's still alot.
Yes, but now you really only need to search for 21 biomes instead of 36. Not too shaby, eh?

Okay, fine. I just want the advancement. How do I search for them?
I suggest making a list first. Take every biome on the "What biomes do I have to explore" section and put it all down on a piece of paper. Cross off every biome you know you have visited. Then, go to the ocean. You should search for one unless you know where one is already.
Or, if you have loaded an application like WinRar, you can view your progress in the stats.json file in a world. If you think you forgot a biome, you can view it there to check.

Does elevation matter?
The biome will still register if you are underground or super high up.

Why the ocean?
Chances are, every single biome is not going to be found on your spawn island, no matter how big it is. You must go to other islands where the climate differs from yours. Maybe you spawned in a warm climate, and there are no cold biomes nearby. Also, a good portion of the biomes are found in the ocean, like:
Beach, cold beach, deep ocean, mushroom island, mushroomislandshore, ocean and stone beach.

What if I use Xbox, PS, Pocket Edition, or any other platform that is not PC?
Lucky you, the load is much lighter. You only have to explore 17 different biomes to get the achievement (not advancements yet!). These can be any of the 40 biomes available, you just need to enter 17 different ones! Not only that, but the game saves each biome you enter on every world! You could even create 17+ new worlds, spawn in them, and delete. You don't even have to walk a meter to get it!

I don't know which biomes are which. :(
Easy. Just pull up F3. The biome you are in will be displayed in the top-left corner area.

Did I miss anything? Confused? Comment below what questions/comments you may have.