What's your level of sewing skill and what's your definition of "make"?
For myself, I started out just wearing a couple of pseudo-Victorian items as part of my regular outfit, and it seems like every con I go to, I add something else and it's turning into a costume. So far it seems to be centered around a poet blouse (from a thrift store), a velveteen jacket (from a sadly defunct women's clothing chain), a cincher corset (from a local corset-maker who sells her stuff at cons) and some slightly piratical lace-up boots (from Zappos). I say it's edging into costume territory because I got asked to pose for a picture this year at the con.
In fact, a lot of the people I see in steampunk garb at these things are wearing a mix of well-made artisan items, off-the-shelf new or vintage real clothing items (i.e. stuff made to be worn, not commercial costumes that are cheaply constructed), and stuff they made themselves. A few people wear outfits that they made all on their own, but they tend to be the professional prop-makers and tailors/seamstresses who also sell their stuff to other costumers.
Some people construct a costume around a character or alter-ego that they made up, so that they aren't wearing a random assortment of parts, but have a costume that comes together functionally and makes sense. I have an idea for a field scientist or naturalist character, which is based on my own real-life interests and expertise. I'd probably wear something like a Victorian hunting outfit that would be comfortable in the wilderness of whatever climate I was in, and carry a bag and a pick for rock samples, a loup or magnifier for examining specimens, a sketchbook, and maybe some little cages to catch animals (most likely the weirdest plush toys I could find).