Turn on the spectrophotometer. Many machines must be on half an hour or more before use; check your spectrophotometer's manual or consult the technician of the lab. Turn on the monitor and printer associated with the spectrophotometer, if applicable.
Select the proper wavelength. You should generally use a wavelength of 600, but the most important thing is to consistently use the same wavelength for each sample.
Fill one cuvette with water or the broth your bacterial cells are growing in to serve as the blank.
Open the lid of the spectrophotometer, then insert the blank in the cuvette slot. Make sure to orient the cuvette properly; some cuvettes say "Front," while others simply have two frosted glass sides and two transparent sides. Place the transparent side or the side that says "Front" facing out towards the front of the machine.
Shut the machine's lid, then select "Read Blank" or similar. After the machine reads the blank, remove the cuvette from the spectrophotometer.
Fill a cuvette with the sample you wish to measure. Place this cuvette, with the proper orientation, in the spectrophotometer. Close the lid and select "Read Samples" or similar.
Record the results of your sample. If your spectrophotometer is equipped with a printer, send the results to the printer to print a copy.