How to Clean a Shirt Collar
To clean a shirt collar, start by spraying a stain remover, such as Resolve or Zout, on the affected area. Then, take an unused toothbrush and scrub the area to remove as much of the stain as possible. Once you've removed most of the stain, let the remover set for 30 minutes. To finish, wash your shirt as normal but check that the stain is gone before you dry it so the dryer doesn't set the stain into your collar.
Spray your collar with the stain treatment. Generously spray a stain treatment of your choice onto the collar. Make sure to focus on the stained areas. Examples of some great stain treatments are Resolve, OxiClean, Shout, and Zout. Scrub the stain. You can do this by rubbing the collar against itself. Or, you can use an old (or unused) toothbrush to scrub the stain. Scrub your collar until the majority of the stain is removed. Then let the stain treatment set for 30 minutes. If it is a stubborn stain, then you may need to let it set for an hour. The scrubbing will help work the product into your stain. Wash and dry as usual. Once the treatment has finished setting, wash your shirt as you normally would. Before you dry your shirt, however, make sure the stain is completely removed. If not, the dryer will set the stain into your collar. If the stains remain on your collar, then use a different stain treatment and repeat steps one through three again. Pour two tablespoons (30 ml) of a dishwashing detergent into a bowl. Use a powdered dishwashing detergent. Mix in one tablespoon (15 ml) of water until a thick paste forms. The paste should have a toothpaste-like consistency. If the consistency is too watery, then just add more detergent until a thick paste forms. Wet your shirt collar. Place the collar of your shirt under cold running water. Make sure the front and back of your collar are completely wet. Rub the paste into your collar. Use your fingers to rub the paste into the stains and dirty areas on your collar. Let the paste set for an hour. Scrub your collar. Use a toothbrush or a bristle brush to scrub your collar, focusing on the stained areas as you scrub. Scrub your collar until the majority of the stain is removed. If the paste is dry, just re-wet your collar with cold water before scrubbing it with the toothbrush. Wash and dry your shirt. Wash and dry your shirt per the instructions on the tag. Before drying your shirt, make sure the stain is gone. If not, the dry heat will set the stain into your collar. If the stain remains, then repeat steps one through three again, or use a different method. Combine three parts baking soda to one part hydrogen peroxide. Mix the ingredients together until a thick paste forms. The paste should have a toothpaste-like consistency.  If you are cleaning a dark or colored collar, then replace the hydrogen peroxide with water. Hydrogen peroxide may bleach a dark or colored collar. Wet your collar. Do this by placing your collar under warm running water. Make sure the front and back of your collar are completely wet. Apply the paste to your collar. Dip a toothbrush or a bristle brush in the paste. Apply the paste to the stain. Make sure the stains and dirty areas are completely covered with the paste. Scrub the stain. Use your toothbrush to scrub the paste into the stain. Scrub your collar vigorously until the majority of the stain is removed. Then let the paste set for 30 minutes to 1 hour. Wash and dry your shirt. Once the paste has finished setting, wash your shirt per the instructions on the tag, or as you normally would. Before drying your shirt, make sure the stains are gone. Dry your shirt in the dryer, or hang dry it. If the stains remain, then repeat steps one through four again, or use a different method. Lay your shirt flat on a table. Make sure the collar is facing up. Place a towel underneath your shirt to prevent any detergent from getting on the table. Pour the detergent directly onto your collar. Use your fingers to rub the detergent into your collar, making sure the stained areas are completely covered. Let the detergent set for 30 minutes to 1 hour. Alternatively, you can use a shampoo of your choice instead of liquid laundry detergent. Wash and dry as you normally would. Once the detergent (or shampoo) has finished setting, wash and dry your shirt per the instructions on the tag. Make sure the stains are completely removed before drying your shirt in the dryer. You can either hang dry your shirts, or dry them in the dryer. If the stains remain, then repeat steps one through three, or use a different method. 
Spray your collar with the stain treatment. Scrub the stain. Wash and dry as usual. Pour two tablespoons (30 ml) of a dishwashing detergent into a bowl. Wet your shirt collar. Rub the paste into your collar. Scrub your collar. Wash and dry your shirt. Combine three parts baking soda to one part hydrogen peroxide. Wet your collar. Apply the paste to your collar. Scrub the stain. Wash and dry your shirt. Lay your shirt flat on a table. Pour the detergent directly onto your collar. Wash and dry as you normally would. 