How to do the corners with sealent tape in a shower cubicle ?

What sort of selant tape?

(I find silicone and a Fugi is the simplest way)

Assuming you mean the silicone sealing strip that you bend in half and peel off the backing paper to expose the adhesive, so that one half sticks to the shower tray and one half sticks to the wall, you have two choices at the corners. Either leave the top strip overlapping the bottom strip, or mitre it at a 45 degree angle by cutting through both strips with a Stanley knife/sharp blade. On acrylic baths and shower trays the adhesive usually fails after a few weeks due to flexing movement between the edge and the wall.