Advice needed, how detailed itinerary Thailand

Hi Guys,

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I would like to make a solo backpacking trip to Thailand somewhere around half December or beginning of January. I would like to plan the trip for somewhere between 4 and 6 months depending on how much i like it and whatnot. The trip would start at Bangkok and from there i have seen a variety of great things i want to do while i'm in Asia, for example learning to scuba dive, learning to surf, making a trip through the jungle, sightseeing the great buildings, getting to know the people and its cultures, basically the full package of what you can get while you are at this side of world.

I don't want the itinerary to be sett in stone since i do want to have the freedom of travelling along wherever i find people that i have a good connection with, for sleeping i'm going to book hostels so meeting fellow backpackers shouldn't be an issue.

So my question is: where do people make a itinerary, and how detailed should it be?

I ordered the lonely planet guide of Thailand to have some specific places i want to visit but how detailed is detailed enough?

Also recommendations for places will be appreciated, the start of the trip is planned in Bangkok and i want to see the greater part of Thailand, but going for some parts into Laos, Vietnam, Cambodia and Malaysia wouldn't be a issue.