Take your measurements. Do this naked, and don't lie to yourself because that's just dumb.
Make a little sketch so you'll remember, or write it down.

Measure: True Waist,
Just above hips
Just below ribcage
From below belly button to xiphoid process

Decide how much cinching you want to have done. 4"? 3"? Also, corsets don't go all the way around. There should be a little space between the pieces in the back, like this:

(or potentially more space than that)

For the pattern: Divide largest measurement (probably ribcage) by half, and minus 2 or 3 inches. Draw a rectangle with that width, and the height being the measurement from below navel to xiphoid process.

Section the rectangle into four sections, with the end section being slightly wider. At the center between top and bottom, draw a horizontal line. Along this line, squeeze the pieces into hourglass shapes by 0.5" on either side. Keep the furthermost edges straight (on my sketch, I initially didn't do that on one edge, so don't you do that).

pattern

Number each pattern piece so you can keep track of its position and side.

Now, pin the pieces to your fabric. Cut them out, then flip them over and cut out the reverse. You'll end up with four of each pattern piece, two for each side of the corset. Number the pieces as you cut them out with a pencil or some other washable marker.

If you're doing the corset all in one color of fabric, you can save time with the end pieces and cut them out like this, with the fabric folded at the end:

Don't forget to leave spave for the seam. I always have 0.5" around the edge of each piece.

Now, pin the fabric together, and sew it.

(pin and sew the orange lines together)

Do if for the other side too.

Sew the bottom edge.

Now, flip it inside out (so the seams will now be on the inside).

Get a big plastic jug, like a juice jug or detergent. It will need to be about as tall as the navel-xiphoid measurement. Cut it into strips, about 0.75" wide. For the ends, you'll need wider pieces, about the width of those areas.

Here's a visual illustration of the sizes you'll need for the boning:

Place the plastic into the corset piece, pinning it into place. Use two strips of plastic for each section, to keep it stiffer. If you use white or transparent fabric like I did, make sure the print or label on the boning only faces one side.

Here's a pic of the boning pinned into place before sewing:

Sew the boning into place. For the raw bottom edge, use ribbon or finishing tape to close it. I used some pink lace, as it would match the ribbon I eventually used to lace it.

For grommets, measure along corresponding edges. Don't place them any farther apart than about an inch, or it will make the corset buckle. I'm lucky enough to make tools for grommeting, but if you don't, you can use eyelets instead.

And that's it! Feel free to ask questions and I'll do my best to answer.

Oh yeah, remember when you lace the back to use two pieces of lace, not one. They should tie in the middle. This way, it's easier to cinch it up.

Please please, if you make one, let me see! Take a picture and post it!

Edited to add:
The talented painter Rachel Bess painted me in this corset from a Dr. Sketchy's session: