Whilst these are relatively straightforward to fit to the inside of your window there are some key issues to be aware of:
1. Think of the material like a tax disc (we all remember those) and you won't go far wrong.
2. Make sure your windows are clean, seems obvious but any dust or grease will cause problems. You can clean with soap and water, windolene (other makes are available) or good old vinegar.
3. Dampen the windows or fit at night if you experience further problems, although this should be unlikely if you follow the list above.