You have your first pattern ready and the next step is to test the pattern and fit on your fit model. Often referred to as a muslin sample, your fit sample should actually be made up in a sample fabric that is the same weight, weave and fiber content as your actual garment, rather than cotton muslin.
Why use "real" fabric?
Cotton muslin bears resemblance to very few real garments. For items that are made up in a lightweight woven (cotton or otherwise), using muslin is just fine. It's cheap, easy to make up and easy to mark on during a fitting. Muslin is also a great tool to use during the pattern making process, for styles that need to be draped, or even just to do a quick test for your pattern.
To get a good idea on how a garment will hang and drape on the body, however, using your actual fabric or similar sample fabric will be much more accurate. A chiffon drapes differently than a crepe-back satin or charmeuse. A 10oz. denim fits differently from a 8oz. A cotton twill will differ from a wool suiting. And on, and on.
In the case of using a stretch fabric, something with 2% spandex will fit much differently than something with 4% spandex. You cannot use the same pattern for a garment of the same style with these different fabrics. For the same reason, muslin fabric has no stretch, and so a pattern made for stretch would end up being too small if made up in a non-stretch material.
A knit garment, such as a jersey dress, could never be made out of muslin either, it would not fit or drape anywhere near what the jersey would actually do. Jersey is knit and muslin is woven fabric.
Another simple reason to use real fabric: It just looks better. You are more likely to be critical of a real fabric sample than a muslin because it's harder for most people to visualize the end product, and the excuse "this will look and/or fit better in the real fabric" will be tossed out often. Without testing it first, you might miss that the back thigh of a pant leg is laying on the body funny or that the sweep of a dress is too big or small.
It saves you time, money, and a headache to get your fit sample right before making up your production sample. Keep in mind that is is not uncommon to do multiple fit samples before the production sample is made. This can depend on how fitted the style is or how complicated the design, and also how much information was supplied to your pattern maker beforehand. When producing similar styles, do a fit sample in one style first, before the patterns for all of the styles are made, so that those changes can be applied to the other patterns.
Contact Xochil for questions about your patterns, fit samples, or production samples.