Simple. You use your baht to buy a foreign currency that you think will increase in value relative to the baht. When it goes up in value you sell it. The other approach is to sell currency that you think will go down in value relative to the baht. When it goes down you buy it back at the lower price.

Of course your evaluation has to be correct to make money. If you are wrong, you will lose money. There is risk involved in currency speculation.