It has been a long time since I bought unbleached, cotton muslin, but I do have some tips for you for making a muslin "block."
You want to match the weight of the muslin fabric to the type of fabric used for the finish product. Eg: if the finish piece is made from a light linen, or lawn, then use a lighter muslin fabric for your block. If you're using a heavy wool, then use a heavier muslin. This is because you want the block to drape properly. If you are making a block for your own, personal body, then you should probably make it with slightly heavier fabric, so that you can take it apart and reuse it over and over to make your own different styles of clothes.
Also, it is really helpful to have someone help you that knows how to properly measure your body. (Not your husband/BF....LOL. Been there, done that.) They will know just how to measure in the right spot, (hips for example are always off,) and it's very difficult to get accurate measurements doing it yourself.
It's also a good idea to invest in a good dress form that you can add/subtract padding as your body changes. The good ones are a bit more expensive, but worth it because you'll have that form for the rest of your life and you can change it as needed.
Good luck with your new sewing adventure and have fun!
