If you're planning to take notes, and refer to a guide at a later time, here are some characteristics you should pay attention to. These notes, plus a good memory will allow you to eventually identify the flower (a digital camera won't hurt either). First, what color is the blossom? Guides generally categorize by color such as white, yellow to orange, red to pink, blue, green, black, and brown. It won't hurt to note if a blossom is light blue, deep blue, royal blue, or purple, if you have a good sense for differences in color.
The next characteristic would be the petals. How many are there, two, three, or more than you can count? Many wildflowers have a countable number of petals, while many garden flowers, like peonies and roses, have "many petals". Note the shape of the petals; are they round, elongated, oval shaped, triangular, bell-shaped or just "odd-shaped"? Odd-shaped can be a challenge to describe, but some flower guides use just this category. To go into even greater detail, note whether the petals are separated or joined at some point.
A descriptive sketch of the male and female parts of the blossom can sometimes help with flower identification. How prominent, if visible at all, is the stamen (the anther and filaments) or the carpel (style, and stigma), the male and female parts, respectively? Don't forget to note the size of the blossom as well. Members of the daisy family, or sunflower family, can vary greatly in size, from variety to variety or species to species.
When a description of the blossom may, for some reason, be insufficient for identification purposes, a description of the leaves may help. Some flowers have no visible leaves, but most do. Are the leaves basal (growing close to the ground) or do they grow on and branch out from a stalk? Do the leaves alternate on the stalk, or are they opposite? And what is the size and shape of the leaf? Leaves can be heart shaped, lobed, or wider in one place (either end or the middle) than elsewhere. The edges may be toothed, smooth, or wavy, and the surface may be shiny, dull, hairy, smooth or rough.
During the Lewis and Clark Expedition of 200 years ago, the explorers sent back many specimens of wildlife and plant life to then President Thomas Jefferson. Obviously as the journey took two years, flower blossoms could not be preserved, nor could the plants themselves in most circumstances. Newly discovered plants and flowers were described in detail in journals, and many of these were given Latin names, a few of which honored the explorers (Clarkia amoena, as one example). It wasn't just the blossoms and leaves that were described, but the habitat as well. If you're taking notes for the purpose of flower identification on a field trip or hike, include a bit of descriptive detail about the plant's habitat and growth habits, in addition to describing blossoms and leaves.
Is the plant a creeper, or a climber? Is it growing in direct sun, or deep shade? Is it a water plant, or growing in a wet area? These bits of information can help you nail down the plant's name, or if you're lucky, you might have discovered a new species. If so, your name, Latinized of course, may become a part of the scientific name of the plant. On the other hand, it might just be a common Canadian dogwood, Bunchberry, or Dwarf cornell. A very pretty flower nevertheless.
Try this out on a walk through the neighborhood (practice on your own flowers at first if you want to), or take a hike in the woods. Once you get good at it, and are looking for the next real challenge, tackle a tropical rain forest, especially one which has not been explored before.