Once you've short sold shares you'll have a trading position. This will show on the Positions tab of the My Trading panel:
In the example above, you've short sold 10 shares of the Over 49.5 points to be scored proposition (as shown by the red -10 figure in the Positions column). To buy back these shares you enter a new order using the Order Ticket - but this time to buy the shares:
This new buy order will be sent to the market to be matched - and when it is matched you will exit your position. The system always calculates a net position, so if you short sell 10 shares and then buy back 10 shares you'll left with 0 shares and no position on the outcome of the proposition.
If you buy back your shares for less than you paid when buying, you'll be credited with a profit. But if you buy for more than you originally sold for, you'll be hit with a loss on the trade.