The plasmids are mixed into a solution containing E. coli. (E. coli normally lives in the intestines of healthy humans and other animals.) This solution is placed in a cuvette that fits into an “electroporation” machine.
In this machine, the plasmids are encouraged (not very gently!) to enter the bacteria cells. An electric shock is sent through the cuvette, and this opens the cell pores wide enough for the plasmids to enter. Think of it as a bacterial torture chamber ;^).
Some of the bacterial cells become “transformed” when they incorporate recombinant DNA. The recombinant DNA, in this case, is the 2-4-kb fragment previously sheared and ligated into the pUC18 plasmid.