A poppet represents a person, so ideally it should look (sort of) like a person. Give it a head, two arms, two legs, a torso. You can make your own outline or you can use the ultimate poppet pattern -- a gingerbread man. If you're doing a spell for an animal -- such as a healing spell for a sick pet -- make the poppet shape accordingly. Your poppet doesn't have to be huge, but it should be big enough that you can stuff it with your ingredients later.
Print out the design in Figure 1 and enlarge it on your printer, and use it as a pattern if you need to.
Take two pieces of your fabric, and place them right side together on a flat surface. Place the pattern on top, pin it in place, and cut it out. Leave a little room around the edges for a seam allowance -- usually a 3/8" margin is good. Remove the pattern, and there are your two poppet shapes. Time to start sewing!
If you've never sewn anything by hand before, don't panic. It's not hard, but it does require some patience. You could always use a sewing machine if you're pressed for time, but most experienced poppet-makers agree that it's worth the effort to do it by hand. Pin the two pieces of material together and stitch around the edges. Leave an opening somewhere, wide enough to stick a couple of fingers in. Turn the poppet inside out, and begin stuffing.