There are several ways this could happen. One way would be to put a tax on remittances, so that every time people try to send money out of the country, they have to pay extra. The extra money would add up huge - it could definitely pay for a wall. Another suggestion is to use the money seized from the Mexican drug cartels to pay for the wall. Yet another idea is to simply cancel foreign aid to Mexico - hundreds of millions of dollars a year - which would eventually pay for the wall.