Use systematic random sampling through the entire region of interest.
Preferential sections may be used.
If possible, take thick sections, so you can use the optical disector or optical fractionator probe.
If it is not possible to use thick sections, you must use contiguous pairs of thin sections for the physical disector or physical fractionator probe.
If the particles whose number you want to estimate are in a monolayer, for instance in a petri-dish or 2-D whole mount, use the disector or fractionator probe.
If the particles whose number you are estimating have dimples or bridges, use the connectivity probe, for example when counting alveoli in lung.