What is the simplest explanation of the ASTM G71 procedures that can be done in a laboratory?
ASTM maintains a variety of publications. To the practitioner, usually the most useful are the test Methods. I have also found the guides (such as G71) and glossaries to be useful background. G71 is not a test method, and it lacks any "procedures", the only answer to your question is: "There aren't any procedures in ASTM G71." This may seem confusing since the Guide contains a diagram which seems to imply that a laboratory test involves two equally sized sample rectangles partially submersed in an electrolyte and connected via an external wire.

Since you are unfamiliar with ASTM publications, allow me to use an analogy which may be more intuitive. Take for example a "Guide for baking bread" which recommends that only a circulating air oven, with an air flow of at least 4 cu. meters per minute and a volume of less than 0.1 cu. meter capable of controlling internal temperature in the range of 50° to 250°C to within ± 2° and that the baker should wash his/her hands prior to starting while keeping the tools, ingredients and preparation surfaces at 20°C. You'll note that my guide (I made it up, so don't take it too literally) completely avoids telling you:


what the ingredients are,
How to mix and process those ingredients, and
how to bake bread.


That is, it is not a "method"; it is a series of best practices (or preferred, consistent practices). When is something like G71 appropriate? It is appropriate when the test needs to be customized to fit the problem. The guide recommends the metal be "as large as possible" (meaning up to full commercial size) while being practical in either lab or field test conditions.

There is also the need for the exposed surfaces to be in the same area ratio, and to avoid edge effects, especially those which will not or do not appear on the "real" assembly. The guide mentions 4 techniques to evaluate corrosion: visual (and photographic), mass loss, current, and potential (voltage). Which of those to use (if any) is up to the individual. It is vague because it can't be specific. It can't be specific because it is intended to apply to a great variety of different galvanic corrosion scenarios.