patterns are normally written as you make/design the quilt.
working from someone elses finished quilt you could measure each size piece and multiply but that would often end on shortage.
If you really want to do it, make it. take copious notes as you go.. fabric prep, pressing directions, assembly (strips? diagonally? 4x4, then 8x8?)
Whose patterns have you used that you give a high 5 to for the instructions? Break that down and follow suit?