Buy it from the owners?
The Constitution requires just compensation for takings under eminent domain - usually interpreted as what a willing buyer would pay a willing seller.
For properties of low estimated value (below $50K?), the Government does not conduct a formal appraisal, and makes an offer based on their informed estimate. The owner can accept that settlement, negotiate with the Government reps, or go to court to sue for a higher payment. For a small home where the barrier will take the back 150 feet of the yard, the home owner might be offered a few thousand dollars.
Hundreds of such cases will have to be settled in the Rio Grande Valley for the new Trump barrier. Some cases have not yet been finalized from the Bush-era barrier program that ended in 2009.
From what I have heard reported, money has been budgeted to reasonably compensate land owners this time around, more so than during the Bush-era Secure Fence Act program. Lessons have been learned.