He could do it by trying to have a bill doing what you say to be passed by Congress, or by issuing an Executive Order. Either way would surely fail, since Trump has demonstrated numerous times is inability to produce any remotely valid legislative document, he just can't focus on the big picture, can't entrust others to do it for him and more importantly, he can't ignore his personal gains.
Who build his hotels, in your opinion ?
I would be glad if that was a joke answer, but it isn't. It's common knowledge since way before Trump came with that wall idea that it would be uneffective as well as a money sinkhole. He only advertised that idea because it's easy to picture it in your mind, and it makes some sense if you don't think too much about it and have an insentive to believe it. It's just a catchphrase.
His goal has never been to reduce illegal immigration. It was to efficiently cristalize the support of the people who feel that immigration is a major issue. Trump himself benefits yugely from illegal immigration, financially and because he can use it as a scapegoat, so I don't see why he would work on reducing it.