Once you have deposit your funds into JOYSO smart contract, there should be ETH, JOY and other kinds of ERC20 tokens available in JOYSO for you to trade.
Click on "Trade" in JOYSO, you can see the upper middle and right section of the page marked as "Place a buy order" and "Place a sell order", respectively. Insert the amount and the price you hope to buy or sell.
The buy section is featured with a green "Buy" button and the sell section with a red "sell" button.
Once you sign and submit the transaction message, you permit JOYSO to match orders for you.
Note that if the total amount of your order exceeds your available asset, there will be a pop up message in red. There will also be a reminder if the price you insert are too off from the market price.