Until you get a feel for it, you might be better to utilize somebody else's pattern, but if you want to do your own, ditto Dave Church. But I'd like to see how he fit eight 15x8 bodies on a sheet of cork. I didn't play with it too much but just cut up a sheet last week and only got seven, and I have a chunk left that I think I can get a teal out of.
So, that being said, to make up your own pattern, go to the art store and get yourself a gianormous pad of graph paper, 8 squares per inch. I just bought one recently and it was around $16 for a pad 22x17. Get some soft lead (HB is good) pencils and a white plastic eraser, like a staedtler mars, and a flexible clear plastic ruler. Around an inch or so from the bottom, and 3" from the right edge (if you are a righty) take your ruler and draw yourself a light rectangle 4" high and 15-3/4" long. You will draw in your side view within this box. At the top of the sheet, one inch from the top, and directly inline with the bottom rectangle, lightly draw a box 8" high by the 15-3/4". (why 15-3/4" you might be thinking. If you get yourself an 8ft stick of cedar, you get 6 decoys with a slight bit of breathing room between them out of the board). Now draw a center line lengthwise thru this box. Your top view goes in this box, but you only will initially sketch a half view on whichever side of the centerline you are most comfortable with. The two boxes end up pretty well spaced to sketch up most any head pattern other than one with the neck stretched forward. The tail will be on the right. If you'd rather sketch with the head on the right, draw the boxes in the opposite corner.
You are now ready to sketch up the decoy. Go thru Keith's black duck posting and pick a pic for reference. Sketch the side view first then you can project important points up to your top box.
Side view:
Make an end point in the bottom box for where the body ends and tail tip begins, 1 to 1-1/4" from the end of your box. Just draw a light line from top to bottom of both boxes. If you are using cork, this will be the end of your piece of cork. Everything from this point on will pretty much be sketch/erase/sketch/erase until you get things looking appealing to you. So sketch lightly, and don't "burn it in" until it's the way you like it. I found that I like to sketch in the tail first. Then find where you want the high point of the back to be, then draw the back from that point down to the tail line. You might find your self sketching it 3-4 times then erase the lines that you don't like. Then draw the front end of the body.
Top view:
Look at enough patterns and enough pictures of decoys and you'll find that they tend to fall somewhere between a teardrop shape and an oval shape. The good thing about this view, is once you decide what you like, it's good for pretty much all side views you'll want to do for a gunning decoy. So, take any particular details from the side view that will come into play when you cut out the top view on your bandsaw and project them up to your top view. Then sketch your, for this example, teardrop shape on ONE HALF of your center line. Make any adjustments for details and sketch in your tail. You can call it done at this point if you want, or you remove your sheet from the pad, fold it along that centerline, and go find a big window (this works best during daytime). A sliding glass door works great. put the sheet on the glass with the blank side of the top view towards you and you can see the half you've drawn thru the paper. Sketch the other half, unfold and now you have a full body pattern drawn.
Heads are a bit more difficult and others more knowledgeable than me can probably do a better job, but concentrate on getting the bill right. Once you sketch up a bill the way you like it, make a template of just the bill or the bill and face up to the eyes and save it for all of your heads. It's about the only part of a the head that doesn't change.
Here is one I did from one of Swamper's pics a few weeks ago.
