Further to the below post regarding driving into Thailand from Malaysia, this post covers the relatively simpler process of driving back into Malaysia from Thailand.
Upon returning you will inevitably approach down Highway 4 (closer to the border the road is also named Thanon Phet Kasem) which runs through the border town of Dannok and goes right up to the border. At times this crossing can be very busy with public holidays, Sundays and between the hours of 4-7pm on weekdays seemingly being the times to avoid. But, as with anything like this, there is a degree of 'pot-luck'. Sundays will often be VERY busy and chaotic because of the many coach trips that will be returning to the different states in Malaysia. On the days when it's not busy the whole crossing can be done in 15-20 minutes. Hit it on Sunday and other busy times and it can take up to two hours with long traffic queues and even longer queues at Immigration.
As you approach Dannok, about 1 km before the border, the road will be closed off ahead and this will detour you through a Customs checkpoint, mostly used for trucks. It's the same checkpoint as when you entered Thailand. Merely turn left, follow the signs for your type of vehicle, follow the path to the right and then turn left back onto the road. There is only the one route to go so you won't get lost.
Now, there are numerous ways in which you CAN (and many people do) do this crossing in terms of how, when and where to park your car but I will describe the process which is, as I understand, the official way and the way which will not create you potential problems.
The things you should have readily to hand are:
2) Thai Immigration Departure Card
3) Carnet Form (Simplified Car Import / Export authority)
4) Copy Vehicle Registration document
In terms of a mental / written checklist, there are 5 things you need to do when exiting Thailand at the border:
a) Return the Carnet form (authorising export of your vehicle), this is VERY important
Then park up your car and:
b) Clear Thai Immigration
Get back in your car and:
c) Clear Thai Customs
d) Clear Malaysian Immigration
e) Clear Malaysian Customs
The process is somewhat simpler than entering Thailand but can still be confusing if you don't know what to expect.
Firstly, it's best to clear your vehicle export. As you drive into the crossing you will see a number of Thai Immigration kiosks on your left and in front. To the right of these (kiosks numbered 7, 8 & 9), but behind and after them there is a kiosk marked 'Customs Form Return'. If the border is very quiet you can often stop at this kiosk or just behind and to the left of it. It's important to do this as Customs MAY wish to see the vehicle, whether they do or not is a different matter. Take your Carnet form to the kiosk (it's advisable to also take your copy vehicle registration form and passport). The Customs officer will rarely wish to check your vehicle and will merely take the form from you. You should be asked to sign the form when your return it AND sign in a book regarding form return. Make sure you do both. Once done, you are clear to proceed to the next steps.
Again, this crossing can be very chaotic at busy times so in my view it's safer to head to the car park before clearing Thai Immigration though many just leave their vehicle where they parked up to return the 'Carnet'. If you wish to use the proper car park, after returning the Carnet form, drive through past the kiosk and about 50 metres after look for the large car park on the right (this is the same car park used when you enter Thailand). Take care as you need to cross the path of vehicles entering Thailand and many drivers, especially those of motorbikes, seem to exercise little caution when driving in, probably being as confused as everyone else! Park up the car and walk back around to the Thai Immigration counters (obviously you need to use the 'Departure' kiosks which are next to the Customs Vehicle Export kiosk you just visited, not the 'Arrival' kiosks for those entering Thailand - believe me, I am told people have done it!). Furthermore, if the border is crazy busy, I have also driven into this car park before handing in the 'Carnet' form, there should be no problem as the car is not far away if they demand to see it, but if it's THAT hectic, they will almost certainly not be interested.
As you face the passport kiosks from the Thailand direction there is a group of them to the left which have a metal barrier around. These are intended for foot and coach passengers. Those to the right (kiosks 7, 8 and 9 again), along the roadway sections, are intended for car / motorbike drivers and their passengers. At Immigration you merely need to show your passport and return the Immigration 'Departure' card which you will have retained upon entering Thailand. However, if you are entering Malaysia from Thailand as a visitor (e.g. you are a Thai resident or resident of elsewhere entering Malaysia) you will need to complete a Thai Immigration 'Arrival / Departure Card which can be found at the desks in front of the Immigration kiosks or can be had from the Thai Immigration office to the left of the Immigration kiosks. Once you (and any passengers) have cleared Immigration, return to the car park to get your vehicle.
You can now drive your vehicle back to the crossing and turn right to head for Thai Customs. Again, take care when crossing traffic entering the border. Drive slowly through the customs checkpoint as you may actually get stopped here. They may well wish to check the luggage compartment of your vehicle and/or scan any baggage you have. At busy times getting stopped is less likely as it just jams up the crossing.
After this you drive straight ahead and approach Malaysian Immigration. This IS a 'drive through'. As you approach the Malaysian side you will see the traffic filtered into 2 or 3 lanes. Again, follow any signage for your type of vehicle and approach the kiosks (cars just drive ahead, whereas trucks and mini-buses are filtered off to the left). When you see one empty with a green light, just drive up to the kiosk. There is no need for the driver or passengers to disembark unless asked (you may be asked so that they can (electronically) take fingerprints at the kiosk. Merely hand your passport and those of any passengers to the Immigration officer and follow instructions.
Directly after the Immigration kiosk (and I mean right next to it) there is a further kiosk. Here you must stop and pay the toll charge for the section of highway you will use next (RM 3.60), if you have a Touch n Go card you can swipe the reader just under the cashier's window to pay. After this kiosk you drive ahead and proceed slowly through the Malaysian Customs and follow any instructions there. You may or may not get stopped.
That's all there is to it, as I said, all very simple IF you know what to expect. However, if I stress just one thing in this post it is this:
Remember to return your 'Carnet' (Simplified Vehicle Import/Export) form AND sign the book at the Thai Customs 'Form Return' (vehicle export) kiosk BEFORE leaving the Thai side of the border crossing!!
If you need fuel there are three filling stations with a few km of the border crossing and I normally stop at the second which is a Petronas.