At his SOTU Address he needs to say that there’ll be a 50% fee/tax on every wire transfer of funds from the USA to Mexico. If the illegals pay the fee, it can help fund the wall. If the illegals don’t want to pay the fee — is it isn’t worth it to them — they’ll just go home.
"All Bills for raising Revenue shall originate in the House of representatives." (U.S. Constitution, Article I, section 7, clause 1).