A tape measure for taking your measurements. Not a solid ruler, not a metal tape measure from the toolbox, but a dressmaker's fabric tape measure.
A friend to help you take measurements.
A notebook to write your measurements in.
Sharp pencils (preferably hard pencils such as 2H) and an eraser.
A metre ruler or yardstick. It’s just about possible to draft with a shorter ruler, but not half as easy or as accurate. I struggled for a long time without a metre ruler but was amazed what a godsend mine was when I finally bought one from www.morplan.com. At the very least, look for something long and very straight that will help you draw accurate long, straight lines, and then measure them with a normal ruler or tape measure. But if you possibly, possibly can - you need the real thing to get it right.
A set square (preferably a big one) or something rigid with an exact right angle at the corner, such as a hardback book. Again, you can buy a real patternmaker’s square at Morplan and again, you really need the right equipment to expect to get a good, accurate draft. If you're watching the pennies the book will do, but it's really no substitute. (A right angle is the angle at the corner of a square.)
A calculator (to prevent brain meltdown). If you’re using inches, you may find the Patternmaking Calculator useful – it’s a calculator that uses fractions! And it’s free!
A Flexicurve (optional) to help you draw smooth curves. Again, you can buy proper curves at Morplan.
A large sheet of paper. If you don’t wish to buy a large roll of patternmaking paper, try using a roll of brown paper or the back of an old roll of gift wrapping paper.
A large, flat working surface. The higher, the better, to save your back!
Scissors. Make sure you use a different pair of scissors for cutting paper and card from your fabric scissors; cutting paper with fabric scissors will blunt them faster.
Sticky tape (eg. sellotape or Scotch tape), for sticking sheets of paper together if necessary.
Tracing paper, both a small piece and a couple of large sheets. You'll need the large sheets to trace, from the draft, a pattern that you can cut up and pin to fabric.
Scrap fabric, which you'll need to make a mock-up of the finished draft. Classically, we use calico or muslin but I recommend cotton drill for mocking up corsets.
Once you have your equipment assembled, it's time to take your measurements.