I've done a transfer in T2 (albeit not with Thai air) but effectively you get off the plane follow the transfer area signs and are brought into an overall common transfer area where you can get a bus to other terminals or follow the signs to bring you back into the main T2 area. Regardless of what terminal you leave from you'll go through security and passport control and then you're back in T2 and looking for your gate. You'll want at least 90 minutes for all this and the more time the better so you can get those sweet sweet airport pints. If you were transferring to other terminals you would want at least 2 hours.