Fixing a roman blind batten over a turning stairwell drop?

[Splodgywoo]

How can I raise myself up high enough to fix a roman blind batten over this window? I can position a ladder (with someone to hold it) from the first stair tread to the top left hand side of the window & I could probably drill the left & centre of the batten up. But how can I get to the top right hand side? The vertical right-hand drop from the top of the window to the nearest stair is 12 feet. It is 78cm horizontally from the right hand side of the window to the end wall. I'm hoping for an easy solution..

I know not ideal, but with someone to help hold you against the window while you balance with your feet on the ledge
Does the window open? So you hold onto the frame with one hand and drill with the other.
This is what the professionals use:

Only the bottom left window opens, not helpful in this case.

I wonder if a stack of books under the ladder foot & my two teenage kids to hold it would work... Not feeling very safe.

Yeah, you could give that a go. My dad made one of those step things out of wood once, it worked just fine.
Whatever you do, as you already know, take your time and be careful. I feel bad for making these suggestions.
BTW I think the window looks nice as is, is it to block sun light out.

The window is one of two, the other is about a 1/3 larger! I have to put a black-out blind on it now, the streetlights outside stop the bedroom from getting properly dark at night. Also I want to create better thermal insulation for the winter time.

I think lots of caution & plenty of preparation & it'll be ok. I can wrap the book-pile around with one of those rug anti-slip mats & then get a teen either side, hopefully that'll be ok.

I love a bit of diy. Thinking about your dad's step I might just make one myself, then I could use it again & again. I could custom make it to exactly take my ladder with a nice recess to surround the ladder leg. The stairs aren't carpeted yet so I can drill the step onto the stairs for extra security. Fab. Thanks for your help!