Spreaders shouldn't be too difficult, draw to scale, make sure they bisect the total angle of the "bend" in the cap shroud
and that the cap shroud
is parallel to the mast below the spreader.
For the rigging wire, again, draw to scale, measure each part and add a margin for error (say 10%). Plan to use Sta Lok terminals (or similar) on at least the lower end of each stay/shroud.
If you want to be cautious, just make the lower shrouds first, attach them to the mast, stand the mast using halyards and temporary lines to support and cut and fit the lower shrouds bottom terminals and secure to rigging screws / chain plates.
The mast will be now firmly in position allowing you to measure the rest of the wires.
I would plan to do it all at once but if this is your first foray into rigging, go the careful route
Disclaimer: I am NOT a rigger, just a DIY boat
owner who has always rigged my own boats.