Disclaimer: I'm a historian with a fondness for acting and costumes, but I do not sew my dresses for the sake of scientific research. Many of my costumes are made based on historic models, but not all of them. Moreover, I mostly use a sewing machine and only do the embroidery by hand.
- First of all, get a pattern for what you want to do. You can draw the pattern yourself (if it's either a simple pattern or you're an experienced tailor), you can buy a full-size pattern, you can change a similar pattern to fit your needs or you enlarge one from a book or from the internet. Whatever you do, you will find that in any case (except for drawing the pattern yourself) you will have to adjust the pattern to your measurements.
- When you're buying the fabric, keep in mind what kind of costume you want to sew. Do you want a soft and flowing material for it, or stiff brocade? Is it planned to be semi-transparent or lined with a matching colour and trimmed with fur? What colours match? What type of fabric feels right for your costume and good on your skin? What kinds of material (silk, cotton, wool,...) did they use at that time, and if unavailable/unaffordable, how can you substitute it?
- Never buy pure synthetic fibre, if you can avoid it. Additions of synthetics aren't that bad, e. g. wool with some synthetics can usually be washed easily without shrinking. But pure synthetics usually look like it.
- Buy enough fabric/trimming/lace etc. Few things are more annoying than fussing around because you've bought a few inches too little of whatever.
- Cut the pieces with enough seam allowance if it's a new pattern you're trying out. Makes adjustments much easier.
- For really complicated things (corsets etc.) cut the parts from cheap cloth first and make adjustments. When it fits you perfectly, use the new pieces as pattern for the real thing.
- The most important rule: Don't panic! Even if it's tempting sometimes ;-)
