The first step is to remove the window regulator handle, the door handle (for opening the door from the inside), and the door handle (for closing it from the inside).
The window regulator handle is held in place with a circlip, and you'll need a wire with a hook on the end to pry it out.
The other two parts are screwed in, and the panel is held in place with plastic clips that you have to remove carefully so you don't break them.
Once you've done that, pull the panel upwards to detach it from the top, and it's removed.
Anyway, download the Opel Frontera manual or ask around to see if anyone has one, because it's the same model as the Isuzu Rodeo and the Isuzu Amigo.
Cheers.