I have taken several costuming classes and have made clothing without patterns before. It is a pain let me tell you. When possible always use a pattern...it will make your life so much easier.
There are some fantastic Victorian and Edwardian sewing patterns out there that could easily be manipulated into a Steampunk outfit.
Victorian era dress and apron costume
Men and Womens Victorian Era Costumes
Truly Victorian Pattern Catalog
Victorian and Old West Clothing Patterns
Now if you do not want to purchase a pattern there is always the option of downloading and printing off a pattern.
Period Clothing Patterns
The Costumers Manifesto
This is one of my favorite sites for history and reference photos.
Corset Construction Tutorial
Videos always make things so much easier.
Another money saver is trying out a pattern before cutting into your actual fabric.
When I am unsure about a new pattern I will first construct it by using a sheet or other cheap material.
Also when you are wanting to cut corners when buying fabric, because I know it can be very expensive, remember to check out your local thrift shops (depending on how large the piece you're constructing you can use an old coat or skirt), ebay (they have some great bulk deals) and fabric stores that are having a sale.