As with anything you want to do well, you have to practice. The best way to learn painting is by painting. Very elementary, yes, but I could read and watch videos all day and still struggle at the canvas. Learning from others, watching videos, and reading books are great and essential tools to grow, but nothing will make you grow best until you get that paint brush in your hand and paint. Yes, you will fail...this is an expectation...should not be a surprise, unless your expectations are well beyond your reach. Take those failures, make them a benefit and learn from them. If you are not learning from them, then this is time wasted.
I have taken a slower approach to restart painting after 14 years, and I have struggled with value. I don't remember being taught about it in school, or maybe I just wasn't paying attention. I have talked in past posts about this concept as it is important to get it right when painting. If not, your painting will be flat and lifeless. In every book I have read getting the right value is more important than getting the right color. Value creates depth (not color), value creates distance (not color), and value creates 3D perception (not color). Color can add mood to your painting but it is value that wins every time.
In my study, the best way to see value is by squinting. Squinting at your subject (closing one eye and squinting the other is better than squinting both eyes) as best as you can to distort the light coming into your eye so that you only see basic abstract values (light, mid-tones, and dark color). This is a basic concept that can help you block down the basic shapes and colors for your under painting.
What is the best way to learn value?
The best way to learn value is to understand that a color can have many variations of how light and dark it is. Just depends on which tube of paint you grab. I recently purchased a gray value scale finder off Amazon (highly recommend anyone to get one who is struggling with value). If you get one, please take it and get it laminated so you can lay paint directly on it without the card ink coming up over time (just a recommendation).
So how do you use the Gray scale finder? The gray scale finder has 10 values on it. Each tube of paint that makes up my pallet are somewhere on this value scale. You have to find where they fall yourself. So I did a study using an old canvas to assess where my paints lye on the scale (see above study painting - find the "X" to see the actual tube of paint color). It was a good exercise, so I recommend you to do it yourself.
The first step in assessing value is to figure out where your tube paint falls directly on the grey scale. You need to squint at it and hold the gray scale card away (arms length) from you (making sure to eliminate any glare - I do this by making sure I paint up and down because I have overhead lighting in my studio - if I painted horizontally, my paint would pick up glare from the lights which distorts the values in which I am seeing). When you squint, you are looking for the "smidgen" of paint on the card to disappear and blend in with the gray scale color on the card. If, when you squint, the paint on the card disappears into the color of the grey scale, you have found that value of paint. If the paint stands out and doesn't disappear, you do not have the right value and you need to augment the color again to make it darker or lighter. Once you find that correct value this is your starting point. You will then have to move it toward dark or lighter values based on your composition. When it comes to painting a composition, you will need to compare this appropriate value with the other values on your canvas. Sometimes, the color could be correct on the scale card but doesn't work on the canvas. So be sure to compare all your values with one another from different locations in the painting.
To go darker you have several options - 1) Add a little of black to get to the next darker value (making sure to always compare your values with the previous ones painted). 2) add the complimentary color to the pigment (i.e. opposite color on the color wheel). The example study above I used black just for an understanding of how black and affect a color. To go lighter, adding white in differing amounts.
I recently discovered, through my reading, that a valued (mono-chromatic - one color only) under-painting initially before adding any color will help you identify if your painting is on the right track. If you can't get the value correct with only one color, you can guarantee that you will not get the correct values for the top layers of color. When you choose this method you will need to make sure your under painting is completely dry (2 to 3 days) before adding color. If not, your colors will mix with the under paint and distort your final composition and it could make it suffer.
A good exercise would be to take an object, squint, and lay down only 1 color. For example, in the paintings below I only used Burnt Umber. I started out with a burnt umber prime of the panel, drew the still life to the panel, then I added the darkest of the dark to the areas I was seeing (using straight burnt umber), then I wiped off the areas of the lightest light using OMS which identify as my light (which is just pulling off all the paint until I see the back of the white panel). No white was used in painting this painting - hence, mono-chromatic term.
Example 1: Piggy - Burnt Umber only; 6 x 6 in., Oil on Panel study
Example #2 Mazie - The Schnauzer (second day): 6 x 6 in., Oil on Panel
Below is the initial picture I used (cropped), the second is a black and white edited photo that helps me see the values in black and white. The last is the study painting using the black and white to check areas that I missed.