I am using Leica M6, in the manual, it said
Expose color negative films for important middle-tone areas and never fear overexposure
Expose b&w film for shadows and develop for the highlights
For the middle-tone areas metering, I assume I should meter using the centre of the frame in an area which is not too bright, and not too dark first, then move the centre back to construct the photo, am I right?
For b&w, I assume it mean to meter on the shadow right? But what is the meaning of develop for the highlights?
For the overexposure, should I keep it consistent for the whole roll of film? e.g. always 2-stop overexposure
Should I also overexpose for b&w film?
"Mid tone areas" is a big vague in this context. I would suggest treating your primary subject as being in the "mid tone" region and work from that.
At the risk of being accused of heresy I would also suggest you consider using a small P&S digital camera ( cheap and used ) to aid metering. Even the cheap ones have good metering modes and can help in making metering decision for film. Some of them also have histograms which can occasionally give some aid.
This may explain Expose For the Shadows, Develop For Highlights to you.
Overexposure is a tricky area, as different films have different characteristics. And if you're not developing yourself, or have limited experience ( or have forgotten a lot of what you knew, like me :-) ) then overexposure is something to be conservative with unless your primary aim is shadows. I'd start here and work from there.
This article may help you find other sources of information for developing B&W film, which is an art in itself.
The statements you read in the manual are the classical way of describing the correct exposure in simple words. Many manufacturers used similar descriptions in the past. These statements are generally true for films and have been confirmed by numerous professionals and amateurs over the years. It is an easy guideline for near-correct exposure in most circumstances.
The digital age has changed most of this. But let me address your questions:
the "Middle-Tones" here refer to the center of the whole range of contrast in a scene. The colour negative films have quite some latitude when it comes to the exposure. Underexposure by one F-stop is undesirable, but never a big problem. Overexposure by two F-stops is seldom a problem. Therefore the idea of this advice is that you put your meter to the "important" part of the scene, that part that you want to show. If you're unsure, overexposure is better than underexposure.
The industry has created "gray-charts" (like the Kodak Gray Card R-27) to simplify generic measurements. These charts have a density of 0.75 log D. (which means that it reflects about 18% of the light) This is the brightness that usually "carries the most content". Put this chart into your scene and measure the exposure on that chart. This will give an "average" exposure which is correct for the light in that scene. However, adaption of this is required if you have mainly whites or blacks in the scene.
This is the general advice to control the contrast of a b&w scene. Traditionally, b&w films are processed and printed by the photographer. So everything is under his control. The problem lies with the sometimes huge range of contrast.
The idea behind this advice is the following: You want enough exposure in the shadows to draw at least something there. Underexposure lets the shadows slip into complete darkness. Therefore, measure "towards the shadows" to get the shadows.
This results in lights that are too white. This can be countered with reducing the contrast in the development of the film. It's pretty easy to reduce the contrast during development by reducing the development time.
overexposure + underdevelopment = reduced contrast
underexposure + overdevelopment = increased contrast
When producing prints from your b&w negatives, it is usually better to print low-contrast negatives which you increase in contrast. The opposite is possible, but renders usually worse results.
If you measure both the shadows and the highlights you get an idea about the range of contrast in your scene.
Here the answer is different for b&w or colour negative films. As the colour negative films are usually processed in a lab, you don't have the possibility to influence the contrast. And even if you are able to process them yourself, it does not deliver the desired results. Prints are also usually created in a lab, where each image of a film is printed "with its own" settings. Therefore, it does not really matter. Newer generations of negative films (the ones after the Kodak Ektar 1000) usually have a huge latitude when it comes to the exposure.
B&W films however, are processed by the photographer. The question also applies only to 35mm film (not sheet-films). Since different scenes usually have a different range of contrast, it is usually desirable to control the contrast "per subject". Therefore it is convenient to use one film only for one subject and choose the setting for the whole film.
Generally speaking overexposure is better than underexposure. This is true for both, b&w and colour negative. This is just the general rule. But if you go for the contrast control mentioned earlier, then No: use the correct exposure for b&w. For colour negatives slight overexposure is ok.
Color: If you have shot digital before this you will come to see that film works a bit differently. In digital photography big problem is overexposing, making highlights non retrievable, while in films it is reverse. Bigger problem is not capturing enough light in darker areas.
To understand why you should shouldn't fear overexposure take a look at this little experiment by Carmencita Film Lab.
As you can see, film won't behave the same if you overexpose for 3 stops as it will as if you'd underexposed for 3 stops. It degrades much faster if underexposed.
Black and white: Not capturing enough details is also problem with black & white (since it's also very similar photosensitive emulsion, it is just that color film has 3 emulsions, rather than just one). That's why you should expose for the shadows, meaning making sure they get properly exposed to capture enough details.
When it comes to developing for highlights, I can't give you a straight answer. As I've come to think of it, when developing film, developing time has bigger impact on highlight areas than shadow areas, meaning it is easier to "overdevelop" highlights if you choose your time incorrectly. That is my experience, but I might be totally wrong on this.
I think understanding of zone system could give you better answers than I when it comes to black & white photography. Some time ago amazing landscape photographer Ansel Adams and Fred Archer have come up with what's called zone system. They basically divided all visible parts of picture into zones depending on their gray level. Zoning out your image before you take it can help you to:
When it comes to being consistent when shooting one roll, that really depends on how you plan on developing film. As you will come to learn there is no one way to develop a film, there are such processes as pushing and pulling, and when doing that your whole roll will be affected so being consistent will get all your photos looking as you'd want to.
Here you can find out more info on push and pull process: http://www.richardphotolab.com/blog/pushing-and-pulling-film-the-ultimate-guide/
I am sorry I couldn't give you a better answer for developing right now. I highly recommend you check out zone system, since I believe it will give you a new way of looking at black & white photography and how you do it. :)