You pay your mission living expenses to your home ward using a donation slip. The online donation slip will have your name listed under Ward Missionary Fund. Just click the Select a Missionary box, select your own name, and pay your regular monthly mission expense.
If you live within commuting distance of the Missionary Training Center, you can live at home and travel to and from the MTC each day. You will still need to pay for any meals you eat while at the training center, though.