Gather all items in one place
First of all, gather all the cosmetic items you have in one place. Gathering all products in your bedroom would be a very good idea. Make sure you get items from all over the house as sometimes things are kept in different rooms and sections.
Separate products in categories
When all the items are kept in one place, divide them into different categories. Items of the same category should be kept in one place. For example, gather all the eyeliners in one place and the lipsticks in another. This will make things convenient for you when you will go through items.
Go through each lot
The next thing you need to do is to go through each and every category or division. Have a look at all your eyeliners, eye shades, lipsticks, cleansers, moisturizers etc and see which ones you need and which ones you do not.
Screen out items that you do not need
The last thing that needs to be done is to screen out the items that you do not need. Carefully go through the items that you use and select the ones you are likely to use in the future also. Thrown the remaining items in trash so that they do not cover any extra space on your dresser.