I see, well if you don’t wanna spend the money, both, Blender and Make Human are totes free, so it may pay to learn the basics of both, I swear is way easier than it looks.
The mannequin I have is similar to these ones:
It came with interchangeable hands, props, heads and… uh… proportions…
Any way, that set is quite expensive, but there are others that are around the $20 price or lower.
But I’m afraid to say, this is a matter of practice, the more you draw the basic shapes the easier it will become to picture that stuff in your head.
Good news is, there are several ways to the same end, one thing you can do is getting pictures of people and draw the basic shapes over them, concentrate on individual parts instead of the whole.
Figure drawing is another thing to do the stuff, find people or pics and just draw them, plenty of tutorials out there, just pay attention as to why does the body looks the way it does.
Try to avoid learning from others drawings thou, there’s nothing wrong with them per se for posing, but if the artist has mistakes, you will inherit those mistakes as well, not to mention that in the case of style you may not know why a body part looks the way it does.
Another thing that may help you a lot, is learning anatomy, Proko has a good bunch of videos on the topic, with very detailed explanations as to way body parts look the way they look like,
But in the end, is just practice I’m afraid.