Choose a field guide (see Resources), and note how it's organized. For example, the Peterson field guide "Eastern Trees" organizes trees based on leaf arrangement, so when using this field guide, you know to observe leaf arrangement when collecting information about a tree. Read the introductory material and a few descriptions to see what kind of information the field guide focuses on. When making your observations, be sure to include these details.
Find a leaf bud on the stem. If it occurs at the base of a single leaf, the plant has a simple leaf. If it occurs at the base of multiple leaves, you have a compound leaf.
Observe the arrangement of the leaves on the stem. Leaves that emerge in pairs, one opposite the other, have an opposite arrangement. Those that occur singly and alternate sides moving up the stem have an alternate arrangement. If leaves move in a spiral pattern around the stem, the leaves have a whorled arrangement.
Describe the basic shape and size of the leaf. Make sketches, take a photograph or collect a sample, if you don't have your field guide with you. If you aren't comfortable with botanical terms, use descriptions that are comfortable for you and translate them later.
Note the leaf vein pattern. If the veins emerge from a single point at the base of the leaf, the leaf has palmate venation. If the veins emerge from multiple points along the stem that runs down the middle of the leaf, the leaf has pinnate venation.
Describe the edges of the leaf. Some leaves have smooth edges, while others have jagged, wavy or wrinkled edges. If you don't have your field guide with you, write down all of your observations so that you have them handy when making the identification at home.
Observe other details about the plant, including its surroundings. Note the habitat and how it grows in relation to other plants. For example, although many wildflowers grow in the forest, some grow in the shade beneath trees and others occur only in sunny clearings. Habitat details can help you narrow down or confirm an identity made based on the plant's leaves.