It's generally called a walk away split. Just take some frames of brood of various ages (capped and open and eggs) and some honey and pollen and set up the nuc. If you want to do an even split then just put half the open brood in each of them and half of the capped brood in each of them. Half of the pollen in each of them and half of the honey in each of them and let which ever half the queen DIDN'T end up in raise a new one.
The only issue with this is that SOMETIMES the resulting queens aren't as good as swarm queens or supercedure queens.
Jay Smith ("Queen Rearing Simplified" and "Better Queens") speculates that the reason that emergency queens (such as these) are sometimes inferior is because they bees can't tear down the cell wall and have to float a larvae out instead where with swarms and superceudre they can plan ahead and build a queen cup for the queen to lay in. The soluition is just to find three or four just hatched larvae and tear down the side of the cell that is on the bottom so they can build it into a queen cell without the cocoons stopping them from building it the right way. One beekeeper I know, uses a 30-30 bullet to break the cocoons. He just pushes it into the mouth of the cell and flares it. Then the bees do the rest.