You need to acquire some photos to work from. They should be about 5 by 7 or so. There are books of nudes published for painters that work well. Mail order clothing catalogues work too, . I used to have a book full of pictures of sculpture that worked well. In this internet age, there are plenty of things online that you can print out that will work.Edwin Muybridges book "The Figure In Motion" is also good.The idea is figure against a plain backdrop. There are websites full of figures just for drawing, but they charge a fee. If you can cut your references out, that is best. Here is how to go about the memory drawing exercise.
Lay the photo of the figure out on the table and lay a piece of tracing paper over it.Trace a square onto your paper that is exactly the size of the photo. Remove the tracing paper and then stare at the photo for about five minutes trying to remember the shapes and proportions. Then put the photo face down, out of reach and cover it with a book, so you wont be tempted to "cheat". On your tracing paper you have that square exactly the size of your photo, in it you draw the photo as well as you can remember it. Time this, give yourself,about five minutes. An old fashioned kitchen timer works well for this. When the time ends lay the drawing on the tracing paper over the photo and see how close you have come.
Then repeat the process,look again at the photo, draw a border on a new piece of tracing paper, put the photo away, draw and compare. Do this repeatedly, you will find that each time you do it you will get closer. The ability to do this increases with training.
When I was studying with Ives I went through periods of time where I did this every day for about half an hour. I haven't done it in a long time, but I can assure you it is an excellent practice if you can summon the discipline to make a daily regimen of it for a while.I can't say I ever thought it was much fun, but it is worth the trouble. It is interesting that you CAN train your visual memory.