Green Book, English Translation Letter thereof from Land Transport Office - I had mine done in Songkhla and they mistranslated the Thai letters - which wasn't a real issue. Then have romanised translated 'number plate' sticker made-up before crossing into Malaysia. I believe that you now may be able to get a permanent romanised number steel plate made up here in Thailand that should mean this process is simplified now. Better check at the land transport office.
Important point when crossing in Malaysia is to buy insurance at the border, to include enough time for travel to S'pore and back to Malay/Thai border.
I neglected to do this as the insurance wallahs at the border were having a long lunch and had to buy it in Penang where it was seriously more expensive.
Also when you arrive at the S'pore entry point they will check to see how many days your insurance in Malaysia has left to run and will deduct 3 days off the remaining portion [which is what they assume will take you to return to Thailand riding through Malaysia on the return leg] then those days remaining will determine how many days they will allow you and your bike in to Singapore.
Issues about riding in Singapore are best covered elsewhere, suffice to say the electronic toll system is a hassle that needs some thought & planning.
Riding Malaysia - be aware big bike theft is a real problem even at hotels/shopping centres with supposed gate/security guards and CCTV.
BTW the ride from the Eastern Thai Malaysia crossing point over the mountains via Gerick to Butterfield is sensational - 4th gear roll-on, roll-off sweeping corners.
Crossing back in to Thailand try the little-used Bentong crossing point as the roads are great & interesting.
Make sure that the Thai customs enter your bike back into Thailand on the computer, as they are idle about this and will cause you problems if you don't insist. The road from Betong to Yala is great [see Captain Slash's reports about riding this road]. Make sure you fill up with fuel either just before you leave Malaysia or in Bentong itself - don't do what I did & end up looking for fuel in a muslim market along the way, as this is still bandit territory and certainly don't aim to stay the night in Yala. Betong is fine as it is a 'destination' for Malays seeking a night of what they quaintly term "hanky-panky" - tho definitely on the seedy side.
Otherwise it's a really great opportunity to visit a much overlooked part of Thailand, tho do read Slash's reports as he had no trouble and clearly enjoyed it.
Look forward to reading your ride report as things may have changed.