What is the best way to safely transfer my guitar to Thailand?

What kind of guitar? What body style/size? Value? Do you have a hard shell case? A well padded travel gig bag? I have flown to and from Thailand a few times with different guitars. I have never checked one as luggage and I don’t recommend that you do unless it’s in a high quality flight case. My friend made that mistake twice with his Martin D28. The first time the neck was cracked. It was repaired. The next time it was snapped right through. It was in a hardshell Martin case both times. I would not even try to travel with a dreadnaught. Mine were parlour and OO size in a high quality travel gig bag which fits it the overhead in the cabin or in the flight attendant’s closet also in the cabin. I’m sure a OOO would be fine too but nothing bigger than that. Hope this is helpful.

I sent mine in a flight case labeled as fragile and it arrived fine through Emirates.

Soft gig bag in the overhead or in the hanging closets in 1st / business class.

I shipped a lute in its hard case with stacks of bubble wrap and other padding in a carton. It was fine

Ship it UPS and insure it.

If it's extremely valuable ($3500+), I would buy a seat for it.

For lessor guitars, best to just check it. Chances are good it'll be fine.

Very important: Loosen the strings until they're slack.

If the guitar sits outside where it is really cold in the winter, avoid the temptation to open and check it out as soon as you claim it. Let it warm gradually to room temperature, or you risk checking the finish.

I have done it twice in a hard and a padded soft case as well, no problem. You will need to ship it in oversized luggage so it will be delivered to a different spot then your other luggage but close by

Carry it on board if you care about it at all.

There is no safe way to transfer it in a box. I would Put it in a hard flight case and check it in as oversized luggage. That’s what I did with mine.

Check it in as luggage .........