First, get to the third Bonfire in the forest of fallen giants. To do this you must get the key from the area boss. Nearby the bonfire (soldiers rest) is the giant tree that grows the seeds and an npc invader, Armorer Dennis. Do not kill this invader or you will have to use a bonfire actestic later on to bring him back.

Once you have lit the bonfire, run to the tree and grab a seed from the tree's left arm if there is one. After checking, let Dennis kill you. You'll respawn at the bonfire. Run to the tree again and again. Each time Dennis invades there is a chance for a seed to spawn. Make sure not to kill him or you'll have to burn an acstetic! The seed can spawn in front of you the moment Dennis invades or after you respawn, so always wait till Dennis is there before dying to ensure you don't miss a seed.

Once you have the desired amount of seed, (ahem) simply warp to another bonfire. Told you it was easy! I got around 20 while making this guide, so you should have plenty. Now go out there and troll some invaders!