My beloved husband does the growing part so I can't really help with that. You get a plant, or maybe seeds, I'm not sure, and dirt and water and sunshine. Then you grow it a long time while your wife saves her scraps all year for sachets. Then one day you see buds, and just when some of them start to open all the way - but not all of them, because then the lovely scented oils just dry up - you cut them and bring them inside in one heady, lovely bunch.
OK, now this is the part I do know about. Bundle them into bunches about as thick as your wrist so they aren't too crowded. Rubber band the stems, pretty tight, because they lose some volume as they dry and you don't want them falling all over the place.
Slip another rubber band through the ones holding your bunch, then loop it by pulling one end through the other to form a hanger.
I cut the ends straight because I think it looks really pretty that way but it doesn't serve a single purpose I know of. Hang them in a dry place out of sunlight - our garage is the best place here. Put a piece of newspaper on the floor beneath them to catch the flowers that fall as they dry - you don't want to waste any of these fragrant beauties! In about a week they should be crisp but still have their color and fragrance. Take 'em down, it's time to separate the buds from the chaff.
Put the bunches in an old pillowcase. I have best luck doing this one at a time. Then rub gently, like you are rolling the bunch, and the flowers will separate from the stems. Collect the flowers in an airtight container and don't forget to shake out your newspaper and use those too. I store in a Mason jar and keep it in a dark cupboard. I have heard the stems can be tossed into your fireplace to add some nice fragrance. I haven't tried it ... don't get me started about all the rules my town has about what you can and can't put in your fireplace and when.
Now, on to the sachets! This is the quickest, easiest way to make sachets that are still dryer-friendly. The key to this is pinking the edges, which means cutting them in a zigzag to keep the fabric edges from fraying, because fraying edges would make a mess of your dryer. Yes, you can sew it like a traditional pillow, right sides together, turn, fill and sew the hole, but this is much easier and works just as well.
You can use pre-pinked charm squares, you can cut your fabric using a pinking rotary cutter if you're fancy like that, or you can use pinking shears. I like them no larger than 5" on a side and no less than 3".
Keep the lavender packed down away from your unsewn edge, and then sew the remaining edge, being sure to cross over your original stitches, and backstitching at the beginning and end.
For the longest life as a dryer sachet, toss them in when the clothes are only just damp, then store it in a dark place until your next load.