This reddit post is an excellent guide to creating shapes for Second Life:
Sorry, but this post is anything but an excellent guide to creating shapes. It is a good explanation about human anatomy/body forms in general, but that's about it then already. It's like telling a total newbie who wants to bake bread "to bake bread you need water, all purpose flour, salt and some yeast, easy peasy have fun and enjoy!" and that's it. The whole information about the process' details is totally missing.
Meaning if you give this explanation to somebody new wanting to create a female hourglass or triangle shape it will just keep them scratching their heads in confusion, because it explains the theory without putting it into action and real numbers, therefore it is useless to them.
This post is great for experienced shape creators, who are able to fill in the gaps by their own experience, but that's all it is about due to omitting many important bits of information for newbie creators.