You have to specify the price level at which the stop order will be activated. In case of a sell stop order, your stop price must be lower than the current market price.
Then set the limit price — the lowest price at which you are willing to sell your cryptocurrency.
Finally, enter the amount of cryptocurrency you want to sell, and click Sell.
In the popped-out window, click Confirm.
This stop order will appear in the Open Orders tab and will be executed as soon as the market price reaches the specified conditions.