You are not expected to pay tithing on funds you receive for your mission. You should pay tithing through your home ward on any other income. Contribute fast offerings each fast Sunday in the ward where you serve.
If missionaries receive income in, say Euros, but their home ward is in a country that uses Dollars, how does paying tithing to the home ward work? Do they have to calculate the conversion rates and convert their euros to dollars before sending it home to be entered? Or do they just pay to their mission ward in that situation?