First and foremost, in order to use the services that the market provides, you will need to have a Steam account that has had Steam Guard enabled for at least 15 days. If you wish the transactions to be instant, the Valve Steam Guard would have had to have been active in the last 7 days. With that out of the way, we are almost ready to start but first, we need to connect our Steam account with the website itself which is relatively simple, just click the "Sign in" button on the site and enter your information. If you don't have a Steam account, you will have to make it.
Once this is done, you will see your Steam inventory, simply choose a game, and all your cosmetics for it will appear. It is important to note that some cosmetics will be grayed out and won't be eligible for sale. This is usually due to the website having too many them already. Think about what skins you would like to sell, and select them by clicking on them. As you add skins you keep selecting the skins, they will be moved to the right side of the screen and a button will display how much your selection is worth. Once you know you are done, simply click the green button and you will get your cash deposited into your account instantly.
From there you can withdraw your funds via the payment method that you like or know and trust. How fast you get your money differs on a case by case scenario and is based on the payment method that you have chosen, however, most of them are completed instantly or in a couple of business days.