And this is how I did it.
Find a piece of timber long enough so you can screw the top piece of window frame to it.
This is only to support the frame while you fit the glass/seal to it.
Get all the frame sections, frame screws, screwdriver and a soft faced mallet to hand.
Fit the seal to the glass and spray the frame section with silicone lube.
Place the glass/seal into the frame and tap one end to get it started. Don't be timid with the mallet, you can hit it fairly hard.
Work your way along hitting the top edge of the glass pane until you reach the other end, making sure the seal goes in straight and evenly.
You should be at this stage.
Don't go any further as you'll chip the glass on the screws and have difficulty unscrewing them, which is the next stage.
Take the screws out, and tap the glass fully into the frame section.
Cut the seal so you can get a tight bend with a sharp knife, spray with silicone lube and fit one end frame section (making sure it's the right way up), giving it a couple of taps with the mallet.
When it's fully seated fit one of the frame screws. They're the larger ones, the smaller ones are for the window fixings to the frame. Don't mix them up or you'll crack the glass when fitting the completed pane to the frame.
Do the same with other end piece. Cut the seal ready for the last piece of frame.
Now the slightly tricky part. Spray the bottom frame section with lube and fit to the glass making sure the seal is straight and even.
When it's almost fully on, fit a frame screw. This will help prevent the frame popping off as you fit it.
Tap the frame onto the glass and then fit the last frame screw.
Now you can sort out the seal with a flexible filling knife and a plastic tool. I used one out of a trim tool set. Use plenty of lube to stop the seal being nicked by the filling knife. Trim off any seal sticking out between the frame sections. I will add a bit of flexible seam sealer to the frame joints when it's all back together, rather than get in a right mess during assembly.
Do the other two panes and then refit them to the frame.
It will take about as long to read this to do each one.