How thin? Well, I can't watch you doing them so can only guess you may be applying too much pressure. On a longer pointed petal like a daisy, try holding the metal ball like a pencil, with the ball half on the edge of the petal and half on the foam cel pad. start at the top right side and go all the way down to the pointed tip, stop and go back up to the top left side and do the same thing, stop at the point of the petal. If you are rolling the paste thin enough before cutting your daisy you may not even need to thin the edges. I use the plunger type cutter which veins the flower as it cuts it. If you don't have that type use either a Dresden tool or the dull edge of a butter knife and make a line from the top to the bottom of each petal directly through the center. You want an indentation but don't press too hard or you will cut through the petal.
As for the petals not cupping up, if you don't have a flower former here is what you can do. Use a drinking glass, take a square of foil wrap just large enough to fit over the mouth of the glass and about halfway down the sides. Mold the center of the foil wrap over a rounded object, something like a tennis ball or an orange. Lay the foil over the glass with the cup shape fitting inside the glass, smooth the foil down tightly around the sides. While your flower is still fairly fresh, stick the wire directly into the center of the foil and let the flower settle in, rearrange any petals that may fold or twist out of shape. Let the flower dry in this shape for at least a few hours, overnight is even better. The next day loosen the foil from the sides of the glass and carefully lift it up from both sides until it is free of the glass. Turn the flower over into the palm of your hand and slide it away by pulling the foil off from the flowers wired stem. Carefully lay the flower petal side down on a piece of paper towel to air dry on the back. When it is completely dry it is ready to dust with petal dust and steam to set the color. If you want your petal more closed as in a bud, make a large hook in the end of the wire, stick it into the edge of a block of styrofoam, move the foam to the edge of your table or countertop, weight the center of the foam with a can of soup or something heavy enough to keep it from tipping over. Suspend your flower so it is hanging free in midair and not touching the table top.
I recommend the Potpourri book over his other books only because it has nothing but gumpaste flowers. If you want to learn other techniques of decorating his other books are very good as well.