hi Kat, the best thing is to go back to Howdens and they can order you a fitting kit with template etc.
There should be 4 long screws with the kit and plastic sliders which may still be on your inner door.These need to be taken off and fitted to the decor door by small screws.You can then line the decor door up to the inner door.When you are all square,you fit two of the long screws through the top two holes in the control panel into the decor door and then the other two long screws go through the inner door half way down into the decor door to hold it on.
With the fitting kit it is quite self explanatory.Hope this helps.All the best.
FOXY, September 2008