There are lots of corset patterns out there both free and reasonably priced. But the free patterns, although attractive as they cost you nothing, often are incomplete, low quality or have badly written instructions. So here's what you need to look for in a free corset pattern!
First, and this sounds silly but it happens; are all the pieces there? If the patterns incomplete or inadequately labeled you could end up spending time and money on materials only to find your beautiful new corset is completely the wrong shape! Pieces that aren't labeled are often hard to match up in the correct order and even get the right way up! If somethings missing it can also be impossible to tell. So if your not sure use a different corset pattern or make a quick mock up (something you should be doing anyway with a new pattern).
Look at the lines and curves, measure the bust waist and hip areas to establish how much smaller the waist is to the bust etc. If they're very close it's going to be a very 'up n down' corset with no dramatic curves and is either an Elizabethan corset, a mens corset or very badly drafted!
So you've found a lovely pattern and you believe all the pieces are there, what next? Well if you don't know how to make a corset and this is your first your going to need detailed instructions, does it have any? It's very unlikely. So your going to need to buy some extra literature anyway. Typical huh? Typical but necessary as you won't get far without instructions. A lot of the books out there also have patterns in them, another reason your free pattern is a false economy. But if you know how to insert a busk, properly bone the back opening so the eyelets don't pop out and sew in lining to stiffen your corset then a free pattern is a real money saver.
The last thing you need to know how to do to get started with your free corset pattern is called 'scaling up'. Its most likely your pattern will be a scaled down drawing that you'll print out and need to make bigger. Hopefully if you have a scale on the drawing, or better still a grid, you'll be able to use some squared or 'dot and cross' pattern paper to plot your pattern pieces on. If you can find a corset pattern that prints out at full size this is even better as it will save you an awful lot of time.