well, from my experience for kastam, gonna talk a bit long about it,

if u drive in by urself, u going to prepare

1. Passport
2. Garan original + owner/owner letter and chop by balai polis

when u reach kastam,

1. fill in the white card (like the one on airpork) and chop your passpork. after chop, they will left 1 small pc for u. this you have to return when u go back to malaysia. return to thai kastam.
2. car insurance is recommended, u can do before the kastam u will see a lot on road side.
3. car will have 2 form. 1 is to declare things like passengers, driver(s), owner, etc. this u have to bring to chop passpork and the declaration together.
4. another car form is Kastam Declaration Form or something like that. This is very important and u cannot lose it. YOU NEED TO RETURN TO KASTAM THAI WHEN U GO BACK TO MALAYSIA.

if u fail to do so, your car will be blacklisted. will kena denda THB 10k on your next trip.

sekian

my family got 1 car kena. so.. but just 1 car la. jus dont drive into thai again will do.

if u no kena u lucky la. or we just plain bad luck