What are some effective methods to practice drawing anatomy, preferably in 15-30 minute time increments? Looking for ways to structure a session of practice effectively.
The reason drawing the human form is such a challenge is how our brains work, we have a symbolic representation of what humans "look like" in our minds but that is not how bodies actually look in real life. So the struggle in life drawing is to draw as the eye sees the form, not as the mind records it. Understanding human anatomy and lots of practice (which includes lots of failure in the process) is the key.
Get a good book on anatomy and drawing technique. We used "Drawing the Human Form" by William A. Berry at my college. There are others. These will show you the relationship between the structures of the body and the forms they create. The skeleton, the musculature over it, and the skin that wraps it, they define the form a body takes. How these forms react to light, where highlights and shadows fall, that is what you want to put in your drawings. Studying these books will help you understand that connection.
Attend life drawing sessions. Nothing beats drawing a live human being for what you are after and few things are more challenging. Often there will be a teacher or other artists guiding these sessions that can work with you to correct your technique. Try to be loose, use soft drawing media like charcoal on cheap paper, don't be fussy. Also pay attention to where the light is coming from in relationship to the body and use that to model the form. You could try getting friends and family to pose for you, but that can increase the challenge as it becomes more personal.
If you can, take a course on drawing the human form at a local school or artists collective.
Being effective at drawing anatomy, is practicing often.
Maybe start with one body part at a time. You'll find body parts that you find more enjoyable. Understanding skeletal/muscle structures of the human body would be a huge bonus. Wikipedia would definitely have information about human anatomy.
For quick sketches/drawings of anatomy I use: unsplash.com or pixabay.com
Unsplash has freely usable images, with a lot of human subjects, full body subjects.