Accurate measurement is important. Measure twice (or more) and cut once.
Steel tapes do not stretch. Cords, fabric tapes do stretch or confirm to shapes and might not get a good reading.
Get help from someone when measuring. This will keep the tape taut and will give you an accurate measurement.
Not everything is square. Gravity works! Floors sag. Measure from the top of the windows to the floor on the left, right and maybe even the middle. Do the same for the distance from the top of the window to the ceiling. It really helps.
Double Triple Quad Windows
Bay Windows with a Soffit
Bay Windows without a Soffit
French Doors or Sliding Doors
The width and length of the mattress.
The drop from the top of the mattress to the floor.
The drop from the top of the box spring/foundation to the floor.
The thickness of the matress (the same as the drop from the top of the mattress to the top of the box spring/foundation).
Note if you have posts, rails and the locations. This will help you decide what kind of skirt that might work for you.
What shape is it. Rectangular, Square, Round/Oval
Measure the height, length and width. ( Diameter or similar for round/oval tables).