If you are going to practice in front of a mirror you would need to know what to practice, with the idea being how close is your image in the mirror to the pose in the book. So you could look on line for poses you like, go to a used book store and find a photographers book on posing models [My suggestion], a few fashion magazines. Or best yet find a photographer that is good at posing people, I have had a few models comment that after the shoot they had felt some aches and pains.
The goal behind a pose is to make the model or subject look as pleasing as possible, using the knowledge of the photographer to accomplish that feat of camera magic. For a rotund man you would lean the subject forward to reduce the appearance of a large tummy, you would also have then stick out the chin some so the double and triple chins are reduced, with a bald head you would reduce the lighting at the top of the head to reduce shine off the chrome dome, might use a lower camera angle.
Now on to the typical model, the best shape to create in a models body is an “S” curve, done by lowering one shoulder, usually the one toward camera, also has the effect to lengthen the appearance of the neck, and to raise the hip on the lowered shoulder side. This should create the “S” curve in the models spine.
Now some other tips, place the models hips on an angle to the camera [will look slimmer], don’t point knees, fingers or toes at camera, they will appear shorter if you do. Bottom line if it’s a joint bend it, if it twists twist it, best eye position is to look where your nose is pointed, rather then locking your eyes to one side.
Wish you great success!