Press the leaves by placing them on some sort of absorbent paper - newspaper, paper towels, tissues, etc. Then place these under a stack of books or in a leaf press. After two weeks, the leaves should be flattened, dried, and well-preserved. Evergreens such as spruce and fir are difficult to preserve this way as the needles will fall off. The more ambitious may wish to encase the leaves in epoxy, as this preserves the color and eliminates the need for pressing (meaning the leaves also retain their three dimensional features).
