2 choices really
return into the wall/step
return into means a 90% turn into in other words you cut the board at 45% as if your aiming for the wall and cut a small triangle to fit in the gap so the molding turns the corner and covers up the end grain
other alternative
draw a pencil line coming down the stairs and around the corner till it meets the bit coming along the floor then half that angle is what you cut at
ok just realise they are both level
turn the top bit off skirting turn round the corner 2/3 the height off the skirtings length as in 9" skirting 6" along then down at 45%
as said either return it on itself or maybe use some kind of plinth block.
as for the bottom pic,if your really clever you might be able to make a section of planed timber and carefully scribe it to match and then mould it.so it looks like a continuation of the skirtings .
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