The easiest way to dry oranges or any other citrus fruit you'd like to use is as follows:

• Slice the fruit to approximately 1 cm wide. Too thin and the slices will burn or tear and too fat, they'll never dry.  
  
• Pat as much of the the moisture out as possible using an absorbent cloth.  
  
• Lay them on a baking tray lined with greaseproof paper (reusable if possible).  
  
• Put them in the oven at 120 degrees C for 2-3 hours. Turn them 2 or 3 times.
  If they haven't dried, keep them in for longer. 
• Allow them to cool before using them for decorations. For example: Thread them with ribbons or raffia to hang on your tree. Stud them with cloves and place them in a bowl for a festive 'pot pourri'. Wire them and fix them to a wreath or garland.