Cut the top out of an existing tree and graft in the mulberry you want. The same as we do with persimmons, apples, pears and other fruit. There are many, many threads and info on this forum about topworking. Also, keep in mind that mulberry is like persimmon - either male or female, so to get fruit you may need a sex change operation.
First pic below is a tree about 10 days after I topworked it.
Second Pic is the same tree 17 months later. Yes, they catch back up fast.....