How is a FISH made? 

In this technique, a person’s total set of chromosomes is fixed on a glass slide and then exposed to a "probe", a small amount of purified DNA marked with a fluorescent substance. 

The probe marked with the fluorescent substance finds its corresponding sequence in the set of chromosomes and binds to it. 

With the use of a special microscope, you can see the chromosome and the subcromosomal location where the fluorescent probe was attached.