For anything photorealistic, or really any art whatsoever, you need to start by studying anatomy. If you can, find a book on art anatomy that clearly displays the skeleton and muscle and how it looks and moves under the skin. There's a whole lot to art; there isn't a secret to being good at photorealistic unfortunately. You'll just have to practice a whole lot. If you want a place to start though, I'd suggest Andrew Loomis and his books, which are available for free online. They're old, but they work incredibly well and explain good tricks to drawing proportion correctly.

mhm hard question... I could show you how I make my photo-realistic paintings but I don´t think that a video could answer all your questions. Well all what you need is: first a good reference pic (google or with camera), second show it in colors as your upper screen, third make a sketch of your painting, and after that color it as detailed as you can. That´s one hell of work (didn´t finish my first photo-realistic pic after 30 hours of work). Simply draw what you see (easier said than done, I know).