The key to the perfect fit is to practice on "muslin" fabric. It's an additional step that can be time-consuming but can save you endless hours of thread & seam ripping. Plus it'll save your precious fabric from wear & tear.
It's understandable to jump right into a new sewing project. Has this ever happened to you? You see a gorgeous fabric (on sale!), take it home & immediate start cutting & sewing. Then oops! It ends up being way too tight or you're missing an arm. Then you spend the next 2 hours ripping everything apart. It's happened to us a million times too :-)
Be patient. And practice. Purchase some inexpensive muslin & play around with your project first. Make markings directly on the muslin. Dot seam lines. Do whatever you want on the muslin. Then transfer these markings onto your new fabric piece.
Here are some samples of our muslin prototypes. These are basic garments that can be tweaked into different styles. What do you think? If you have any questions, let us know! Happy Sewing!!!