I am learning the sight size method in Salisbury, and will try to explain a little of it here (as much as I now understand of it). It seems actually not that much different from the comparative measuring we use at Wackers, but the setup is slightly different.
What you need: something to draw, in this photo a plaster cast. A cast has the advantage of not having much colour and clear lights and darks if properly lit. At Sarum Studio, the classroom is very dark. All curtains are drawn, except one or two. This means there is lots of fairly defined shadow and a clear direction to the light. You will need an easel, set at the exact same height as the cast, paper and charcoal, more about which in a later post. The cast and the easel line up in distance from the artist.
If drawing a life size portrait, the easel is lined up with the temple of the model (more or less). If drawing a figure, the easel is somewhere between the model and the artist, so everything you want to get on the paper just fits in.
The artist stands a few meters away from the easel, always in the same spot. Otherwise the perspective changes and measurements will be wrong.You are only at the easel when making a mark, and the immediately walk back again to check.
Now the actual drawing starts: Draw a vertical line in an appropriate place, where you can find (an imaginary) one in real life .In a portrait, this could be along some points of the nose. In a figure drawing, this could be the wall, edge of a chair, edge of your board or an imaginary line in the body. With a piece of string (such as a plumb line) that you hold out horizontally with stretched arms, mark the important heights on your paper. Measuring is done from the far away spot, the actual drawing is at your easel. You don't look at the model when drawing! You have to step back for that.
Next, find the width of the marks you have just made, measuring from the imaginary vertical line. This gives you the exact spot in 2D. Check. Start with easy ones: top of the head, chin, nose, shoulders in a portrait, or head, feet, crotch, shoulders in a figure drawing. You can measure as many points as you need, but at some point you will have to start drawing in the lines as well.
More later on! In the meantime, read Nicholas Beer's excellent book "On Sight-Size Portraiture", ISBN 9781785002038.