"Measure, measure again, set up cut, check angle, start cut, stop, measure again, check angle, finish cut."
You forgot inletting black. The is no way to get a perfect fit without. Measuring things and marking never get it right. Measure big things, like length of pull and rough locations of parts. It is like a good trim carpenter, he does direct measurements on the wood to be cut, he does not use a measurement and then transfer it to the wood.
To install the various parts on a stock is a cut and try process using inletting black of some kind. I use prussan blue oil paint. Some use smoke from an oil lamp. There are lots of options.
I do use shavings, chips and slivers with super glue to fix any mistakes.