Step 1. Go to https://idex.market/eth/qnt. (You will need to log in if you're not already logged in.)

Step 2. Scroll down until you see this box:

Step 3. Change Limit order to Market order.

Step 4. Click on 'QNT' to automatically enter the total amount of Quant you have into the Amount box.

Step 5. Click sell.

If the sale is successful, you should see something like this:

If on the other hand you get a message that says "Order no longer available", then refresh the page and try again.