Before:

Pick a colony and dilute in 10µl water
OR
Take 10µl glycerol stock culture from -80ºC freezer

Protocol:

1. Add 5µl culture to a clean slide
2. Gently pass over flame 3 times
3. Add crystal violet solution (primary stain)  to cover fixed sample.
4. Incubate for 10 seconds (or up to 1 minute).
5. Gently rise with water.
6. Add gram's iodine solution (mordant) to cover stain.
7. Wait for 10 seconds (or up to 1 minute).
8. Quickly rinse slide with Acetone/Ethanol for 3 sec (too long may decolourise Gram positive cells).
9. Wash immediately with water.
10. Add safarin (secondary stain).
11. Wait for 10 seconds (or up to 1 minute).
12. Rinse gently with water
13. Dry slide with filter paper and examine under oil immersion 

Outcome:

Gram-Positive stains (e.g. E.coli, Magnetospirillum) will retain crystal violet and show violet colour under the microscope.

Gram-Negative strain (e.g. Bacillus, Staphylococcus Aureus) will lose primary stain and show secondary stain and show a red colour under the microscope.

Example Gram-Positive: Anthrax bacteria (purple rods) in cerebrospinal fluid sample. If present, a Gram-negative bacterial species would appear pink. (The other cells are white blood cells). (source)

Example Gram-Negative: Micrograph reveals chains, and solitary Gram-negative Legionella pneumophila bacteria found within a sample taken from a victim of the 1976 Legionnaires’ disease outbreak in Philadelphia. (source)