You just need to maintain clearance (aka moat) between the isolated areas. Some components cross the moat (e.g. isolated DC/DC converters, transformers, optocouplers). Planes should be removed from the moat area.
At first, you have to know the regulations, that apply to your instrument under design, e.g. IEC 1010/EN61010.
PCB design tools have a feature of defining net classes with specific design rules (e.g. trace widths and spacings) and also class-to-class rules. The latter are able to check the distance requirements for isolated nets automaticly.