Identify the mono-chloro-pentane isomers first. Remember that the position is labelled from the end of the molecule closest to the Cl atom.
Next identify the di-chloro-pentane isomers beginning with the isomers that have the 2 Cl atoms on the same carbon.
Finally identify the di-chloro-pentane isomers with the 2 Cl atoms on different carbons. Start with 1,X dichloropentanes, moving on to the 2,X dichloropentanes. Consider whether it is necessary to go further, if not, why not ?
The naming convention is simple, X-chloropentane for mono substituted isomers, X,Y-dichloropentane for dichloro isomers. (X and Y being the substituted position(s))
I attach the full set of molecules for you to check you answer against.