One question I am often asked is how I use medium when I paint with Indirect method, so a while ago I made a video demonstrating how, posted below.
For my closed grisaille (gray scale) layer, and or the first couple layers of color over the grisaille, I work very opaquely. I am not glazing, but instead using the previous layer as a guide for putting down color and value as accurately as I can, completely covering the previous layer.
In the final layers, I do work more transparently at times, with a little more medium. Since I am mostly just refining and adding detail to the previous layers, it’s not always necessary to completely cover what I already painted. I only work opaquely when I need to significantly correct an earlier layer.
It also helps to paint the first layers with very blurry/blended edges. I paint very blurry contours in the early stages, and only sharpen my edges later.
Medium recipes, from Lean to Fat:
2 parts linseed, 1 part OMS
1 part linseed, 1 part stand
1 parts linseed, 2 parts stand
I don't use any balsam, drier, or varnish in my mediums. I find that drying times are more predictable and I can get the effects I want without those additives, and by using a simple oil medium I don't have to worry about surprising results in the future, or paint being removed by future restoration.
My videos about painting and drawing comprise an entire classical atelier education online.