You will have to remove the seat from the car. the 126 has electric seat motors, I guess you know that
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gently pry off the plastic side trim pieces, careful, they break easy.
The seat back should have 2 screws under the edge below the net. remove them and the seat back will slide down and off.
the one picture of the drivers seat shows a little bump. with the back off will reveal the button that releases the head rest once it is pulled up. the HR will pull straight up.
the seat back has some motors to move it. locate the wires and unplug. then remove the seat belt rod and assembly ( assume it is like the 123.) from the side of the seat.
should be 2 bolts on either side, remove these and the seat bottom and back will seperate.
the leather cover has the groves the cardboard fits into like the seat bottom, on the sides, across the bottom and where the top fold over the back, it tucks under the edge.
look inside the from the back, there may be 5 or 6 plastic clips that hold the seat fabric tight to the horse hair pad. remove these, and the seat should slip straight up and off.
the drivers seat has the arm rest. from inside from the back there is a large nut, remove this (27mm?) and it should slip out.
doing this from memory with the 126, been a while. I have been taking a bunch of 123 seat apart lately, building up some good seats for my 240. both chassis seats are similar in the way the are put together.
the seat bottom, spring box will fit both chassis. I believe the second generation 126 (86 to 91) may be an improved design, or upgrade with the seat frame.
Have fun working through your project. I have seen maybe 2 green interiors in the PNP yards. not very common.
Charlie