Mark the hook position rather than the frame top: measure from the wire under tension down from the frame edge, and put that mark 15–25 cm above the furniture, checked with a level.
Most crooked pictures are not hung badly. They are measured badly.
Measure the drop, not the frame. Pull the wire or hanging strip taut, measure from the top of the frame down to where the hook will sit. That distance is what you subtract from the frame's intended top edge. This is the step people skip, and it is why the picture ends up several centimetres too high.
Then place it against the furniture, not the ceiling. Decide the bottom edge first: 15 to 25 cm above the sofa or headboard. Add the frame height. Subtract the drop. That is your hook mark.
Use a level, not your eye. A wall's edges are rarely square, especially in an older flat, and the eye follows the cornice rather than the horizon.
Two hooks beat one for anything wider than 50 cm: it stops the frame from swinging out of true every time a door closes.
Fixings, honestly. Into a solid or brick wall, a 6 mm plug carries anything we sell. Into plasterboard, use a proper plasterboard anchor rather than a bare screw, or find the stud. A framed A0 is heavier than it looks.
No drilling? Adhesive hanging strips rated well above the frame's weight will hold a print, but not a large framed piece with glass. Don't gamble on a heavy one.
Finally: live with it before you commit. Tape the paper outline up for a day. Moving tape is free; moving holes is not.