** Note for some reason the You will need section isn't saving what supplies I am adding so in addition to Dried Leaves or flowers you will need a box (like a sturdy cardboard one from a craftstore unpainted or decorated), or a wooden box (likewise unpainted or decorated) Hinged or loosely fitted lids are a plus and Mod Podge or other decoupage glue that dries clear.
First gather your supplies my two examples are made from flower petals and leaves that my MIL had pressed and dried and gave to me. You can buy such things at craft stores but they have more sentimental value if you have collected and dried/pressed them from your own garden or a trip you took (the leaves came from her drive thru New England one year with FIL).
Lay down some protective paper or cloth as the Mod Podge can get drippy and you don't want to ruin your clothes or work surface (tho Mod Podge can be peeled off of most hard surfaces like plastic or wood, or with warm water but it doesn't come out of clothes well).
Take your box and start in one area by spreading some of the Mod Podge glue on the surface.
Carefully select which leaf or flower you want to go and apply it over the Mod Podged area.
Apply another thin layer of Mod Podge over the top of it pressing the leaf/flower down securely to make sure it sticks.
Don't worry the Mod Podge will dry clear.
Continue to put the leaves or flower petals over the whole surface of the box. I had no trouble as the pressed leaves and petals I used in my boxes were like paper applying them on edges and corners. Some of the leaves I had to trim a bit so as not to over lay them too thickly on each other but a little overlapping shouldn't hurt anything.
You may need to take drying breaks to allow you to handle the box in areas where the Mod Podge is not wet and sticky, use your best judgment if your hands where you are holding the box are causing the flowers or leaves to come off because the Mod Podge isn't dry take a break for an hour or so and let it dry a bit before returning to it.
Depending on the box and its lid that you use decoupaging where the lid comes down on the box might require extra care. If you are using one of the cardboard boxes from the craft store you can kind of mold it a little bit so it fits a bit more loosely on the box if it seems too tight. Make sure not to layer your leaves or flower petals too thickly where the lid fits down on the sides of the box or it will be hard to put the lid on or take it off and might damage or tear the flowers off even after drying. Hinged boxes or boxes with loosely fitting lids work the best for this project.
Cover the whole box and the inside and let it dry. You could add a sealer after it is all dry if you really wanted to but I haven't had a problem not using one. The only thing I recommend is that you completely let the project dry with the lid off of the box or if it is a hinged box with the box opened because otherwise it will seal the box closed.
For the future if you ever need to dust or clean the box up a little a damp cloth will be fine but don't prolong contact with water as it will break down the Mod Podge.