fixing external UPVC side window panel frames to upvc door

My hubbie is about to fix our new front door. He's successfully replaced all the other windows, french doors etc but appears to be struggling with this task - I thought it might prove quicker to ask an expert as opposed for him to continue cursing and throwing things.

The frame has come in 3 pieces - a frame each for the two side glass panels and the door. Our house is a 1960 dorma bungalow - no lintels/fixing points above any of the window or door openings. How should he fix the frames together - there are no fixings with the pack that he has been supplied with except those to fix the frames to the brickwork? I hope this makes sense?

If I'm reading it right, normally there should a metal box section inside the frame ( you can test this by taking out the DG unit and drill a small hole) then you just self tap it together, the 1st hole will need be slightly bigger to allow the self tapping screw inside the box section. Have done some but mostly are all difference Some of them clip in together with a middle profile or flat metal bracket at top and bottom depending on the type of frame you have, a photo might help us

Did some joining strips come with the frames? As Masona says, best get a pic