Flowers have from one to four kinds of parts, which are arranged in whorls. You can think of these whorls as circles and, fortunately, for the purpose of identification, these circles always occur in the same order. On a plant with all four parts, sepals are on the outside of the flower. Sepals cover flower buds, protecting the flower before it blooms. They don't look like leaves or petals. Petals are the next circle in from the sepals. The next ring finds a flower's stamen. In the very middle of the flower is the pistil.
Learn what the parts of a flower look like using your reference materials. For instance, pistils have a style rising from the middle of a flower that widens at the top, forming a stigma.
Look for the petal whorl to orient yourself.
Check for sepals between the flower and the stem. On roses these are easy to find, for they tend to stick out beneath the petals of the rose like a pedestal.
Count the number of whorls from the sepals to the inside middle of the flower. If there are four sets of parts, you can now identify them by their order.
Check for a pistil thrusting up from the center of the flower, if you haven't found four parts. This will help you start to narrow the choices of what parts are present.
Look for a whorl of stamens. Stamens are made up of a filament topped by an anther, which is lobed and somewhat spherical. There may be many of them, but none will be in the true center of the flower and none will look like petals.