I'm learning from tutorial videos for 2 years here! The keys it's actually practice doing it many MANY times! It may not look like what you want the first few times,but it'll get better,trust me! This is my recent work and the one from 2 years ago!
Hmm, I think what makes it realistic is depth, form, perspective, and the correct use of values. Using your highlights, mid-tones, and shadows effectively while taking into account proper form and good observations. Your question is sort of an oxymoron because:
depict or treat in a mannered and nonrealistic style.
Well sorry about my poor english, I mean I don't really want to make it realistic, but stylize it to digital painting, because it's impossible I can imitate 100% of the photograph, so I need to stylize it, how to do that?