To prepare gold for refining you should first get a jeweller to remove any large and valuable gemstones. Some refineries will dissolve the gold with chemicals and stones will be left behind in the process. They will charge you a token price per stone for handling: about 1 baht per stone. Be sure to count the number of stones and make sure the refiner includes the stones on the receipt, and that you make sure you understand the receipt if it is written in Thai.
The refiner will weigh the goods and give you an estimate for refining based on the amount of fine gold calculated to be in the scrap. They will verify this by testing at the time of refining.
If you are dealing with a small scrap merchant, such as you can find in the MBK shopping complex, you should insist on watching the entire process, from testing and weighing of your scrap to the melting process.