Does this sound right?
Looks like an old drawing from FHB. Ah, the good ole days.
Not sure what the window jamb material (the mustard-ish colored material in the drawing) is on your window, but if it's wood, Kerdi over wood won't work. If it's vinyl, I don't thinset the Kerdi to vinyl. Poor bond strength. I'll usually try to remove whatever it is and replace it with cement board.
If it is indeed a vinyl jamb an you can't remove it, again, you'll get a lousy bond with thinset over vinyl. I'll spread Kerdi Fix on the vinyl, then use the notched edge of a credit card sized plastic card to spread the Kerdi fix over the vinyl. Just like using a notched trowel to spread thinset over cement board.
Then embed the Kerdi into the Kerdi Fix.
I'd seal the Kerdi to the vinyl window itself (light blue in the drawing) with a bead of Kerdi Fix or an equivalent silane-based sealant. Then after you tile, caulk the tile and sill where they meet the window as shown in the drawing.
Almost forgot. Your drawing has the membrane sitting flat on the sill base, with the pitched sill set over the flat membrane. I like to pitch the sill base itself so the membrane is pitched too. Then install the sill over the pitched membrane.