Easy - on the main homepage you will see a mini portiolio of all the shares you own. Simply select the one you are interested in selling and then select the shopping basket icon. Enter/change the number to sell (by default it's all of them) and then Select "Sell this quantity".
If you are basic member the process is no different except you can't change the amount you sell - you'd have to sell all of them.
And thats it - you'll sell them, and then a message will pop up saying they are sold and how much has gone back into your bank.
You need to keep in mind though - this action only affect the ENTRY you select - if you had made 10 seperate purchases of say Daniel Craig shares and wanted to sell some of them, selecting Sell only sells that entry, not the other 9.