In order to sell your cards, click on the "Collection" tab in the menu and find the card you want to sell. You can only sell a single card at a time, so be prepared for a lot of work if you want to sell hundreds of cards. Anyway, click on the card you want to sell, check the box for the correct card, and click on "Sell". This will then allow you to select the price you want to list it on the market for, so just type in this (in USD) and confirm that you really want to put it up for sale.
Sell your cards on PeakmonstersIt is also possible to sell your cards on Peakmonsters.com, and this website is really nice if you want to list many cards at the same time. The entire process is a bit more cluttered, but in return it's much faster to use once you have gotten used to the UI.
How much should I sell my cards for?Most people probably have no idea what the value of each card it, but it's very easy to check. Just go to the "Market" tab on the Steem Monsters website and find the card you want to check the price of. Click on this and you will see all the listings of the card. If you want to sell your cards very quickly, then select a price that is lower than the current lowest price (even if you only undercut by 1 cent).
If you have some patience, then it might pay off to list your cards for a higher price and wait a little while for it to sell instead.