The stages below are only described in outline. Laboratories which do histology work from schedules which are much more detailed.
Chemical fixatives are used to preserve tissue from decay. This preserves the structure of the cell and of sub-cellular components such as cell organelles (e.g., nucleus, endoplasmic reticulum, mitochondria). The most common fixative for light microscopy is formalin (4% formaldehyde in saline).
After fixing, the block of tissue is embedded in paraffin wax. This holds and preserves the tissue as a block.
The section is cut into a series of wafer-thin slices, each of which is put on a glass microscope slide. The machine which cuts the block is a mechanical guillotine which can be set to cut at a suitable depth for the tissue in question.
Haematoxylin and eosin (H&E)Edit
Electron microscopy is used often as well as light microscopy. This has its own procedures. Its advantage is that it resolves things which light cannot resolve. For example, viruses were first seen by electron microscopy..
Specialised selective stains using immunology or radioactive labelling are now routine. The advantage of using antibodies or radioactive labels is that they stick to specific kinds of molecules. Increasingly popular is tagging with a fluorescent stain, which shows up even if a tiny part of a cell is stained. Immunoinfluorescence is the name of this particular technique.
Media related to Histology at Wikimedia Commons