OK I think I have worked it out, so I will post here how I think it works.
most of it is fairly obvious - such as Quantity is the number of shares and Symbol is the stock code, the ones I wasnt sure about were:
Order type which is 'stop-limit'
Stop price which is the price to start selling at (should be below the current market price)
Limit price which is the price below which you do not want to sell.