Get some fresh lavender flowers together, along with the cloth from which you want to make your sachets, a hammer, and a smooth surface that you can hammer on.
Lay the lavender flowers on the cloth over the smooth surface. You can layer another cloth over top if you's like to experiment with printing on two cloths at a time.
Start smashing the flowers and leaves against the fabric with your hammer. Try to cover the entire plant as evenly as possible.
Carefully peel the plant material away from the cloth. You should have a beautiful print of a leaf or flower!
If you want to, you can touch up your flower with fabric paints. I dabbed on a bit of a mix of pinks and blues together to get a pretty lavender color.
Once painted, set the design with an iron. (That will set the fabric paint and also help set the printed flower.)
Make the sachet
Make a simple drawstring bag by folding down the top edge of the cloth, where you want the drawstring enclosure to be. Iron the fold. Repeat the process by folding down the folded seam and ironing it down again. Make sure to fold it over by around a centimeter or more to have room for the drawstring.
Sew along the bottom of the fold.
Find a similar-sized cloth, and prepare it with a similar-sized drawstring enclosure.
Place the two prepared cloths right side facing each other, lining up their drawstring enclosure area. Sew down both sides and across the bottom.
Turn the pouches right side out.
Make tiny slits in the top layer of cloth in the drawstring enclosure area on either side of each seam. You'll end up with 4 tiny slits.
Thread some string or ribbon through the drawstring enclosure.
Fill your newly made sachets with dried lavender flowers.
Notes
For your lavender, you can either pick and dry your own, or you can easily buy lavender flowers online.
I used a wooden board for my smooth surface, and found out the hard way that it wasn't as smooth as I thought! You can see with my leaf experiments that the wood grain shows up in my leaf prints.