UK New shower

[Puzzled85]

I'm fitting a new shower, 3 walls and a sliding door in a frame. The tray will be tanked to the wall with tanking tape, then tiled. I've read in a few places to seal the between try the tray and tile where the frame meets the wall before adding the door frame so that water can't get behind. That bit I get. What I'm struggling with is after fitting the frame I should seal around the outside of it. But on the bottom corners it will be Silicon over the first Silicon seal. I keep reading Silicon doesn't stick to itself, so is this correct?

 

[Kastar UK]

Manufacturers instructions with the screen have sealing diagram , trim it before if have lumps in corners ?