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If collecting autumn or fallen leaves, stay away from those that are crumbly and papery, as these will break apart extremely easy and be destroyed by the heat. Fresh leaves that are still soft and moist will last indefinitely. Thick leaves, like large succulents, have to be ironed around the leaf design to create a seal around them, not directly on them. Smaller thick leaves (like croton or magnolias) should only be lightly ironed, or not at all, to prevent crushing and breaking. Also consider ironing only the edges or tips of the leaves. Experiment with the techniques that follow, depending on which leaves you pick. Whole stems and branches can be preserved this way simply by ironing the leaves and not the stem. If the stem is herbaceous and soft just iron it like a regular leaf. These are very inexpensive and easy to find. You might want markers, frames, labels, paper for labeling and crafting, and any other craft items for displaying or crafting with the finished project.

Summary:
Look for leaves that are still fresh. Consider thickness. Include stems, if you'd like. Obtain wax paper and an iron. Gather anything else you might need.