To place a limit order on Algodex you must first decide which Algorand Standard Asset (ASA) to trade. Search between Testnet ASAs in the top left-hand search bar. ASAs can be searched for via the asset ID or the asset name.
Once you decide the ASA to trade, find the order selection tab on the right-hand side of the page.
The “My Algo” signing window will appear for you to sign the transaction from your wallet. By default, you will be in “Both” mode meaning you will first take from existing orders on the order book and then make a new order for remaining amounts. In advanced options you can specify if you only want to place Maker or Taker orders.
To place trades, even sell orders, you will need to maintain a minimum balance of algos in your wallet at all times due to the Algorand protocol. The UI does not currently warn you of this. Please keep at least 10 algos in your wallet.