Fit is one of the most important elements in apparel making. Having a garment that fits well makes all of the difference when it comes to the final appearance of that garment on the body. This is why it is extremely important to make a muslin, also known as a sample garment, for fitting before you go ahead and cut out the actual fabric that you will be using to sew your garment.
The word muslin refers to the type of plain, unbleached cotton fabric that sample garments are usually made from, but in fashion design, it is is also used as a term to describe the sample garment as a whole. In the photograph to the right, you can see a muslin that I made using lightweight cotton. This design was a top and a skirt I was making for a friend of the family whose granddaughter was getting married this past summer.
When making a muslin, it is important to select a weight of fabric that closely matches the weight of the actual fabric that you will be using for the garment. If you are making a muslin for a garment that will be made of stretch fabric (knit construction as opposed to woven construction), then it is important to make your muslin sample from fabric that has a stretch that matches the stretch of your actual fabric.
One benefit to making muslin, is that you can make any adjustment you need to make by pinning and marking your muslin fabric. Those markings can then be transferred to your pattern before you go ahead and cut out your actual fabric. I always feel a much higher level of comfort cutting out my good, sometimes expensive, fabric when I already know, thanks to a having done a muslin, that everything is going to fit perfectly.
It may seem like a lot more work to make a muslin, like you are making a garment twice, however if any serious issues arise with your pattern, you will save yourself a lot of time and money by figuring it out during the muslin stage of the construction process. Plus, the more you make muslins, the quicker you will get at making them too. Muslins do not have to be finished on the inside and some elements, such as cuffs and hems, only need to be pressed or pinned in place.
If you want to take your sewing to the next level, executing perfect fit all the time, then you should definitely get in the habit of making muslins.