You can import monomers (often ligands) using File -> Get Monomer...1 by providing the 3-letter code of your monomer/ligand. The resulting molecule will be moved so that it placed at the current screen centre.
Typically, when you are happy about the placement of the ligand, you'd then use Merge Molecules to add the ligand/monomer to the main set of coordinates.
This procedure creates a pdb file monomer-XXX.pdb and a dictionary file libcheck_XXX.cif in the directory in which Coot was started.
A future invocation of Get Monomer uses these file so that the monomer appears quickly 2.