Look for a lot of references about the theme you prefer, for example, people. Once you have a bunch of references spend between 2-4 mins in each one capturing the essence of the photograph. Don't mind if you fail, don't be worried if you are not succeed at first. Give it a try and you will see progress, maybe not in the first days or even weeks, but eventually you will find some of those drawings making you really proud of what you have created.
Art world is hard, you can not compare with masters when you are starting or simply you don't have 20 or 30 years of experience like the drawings of the people you might love. Don't give up and see yourself become the master.
Those are the one I found better between the bunch of work I did. I'm showing you how many tries I did:
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I also have seen people doing this type of exercise but repeating two or three times the same pose or face expression to make sure they learn how the line works in that cases, like "hand memory" and it looks to help a lot. Great exercises by the way!