It is smart to stock up on Antidotes before you enter Pinwheel Forest because there are a lot of Pokemon that have moves and abilities that can inflict poison. As you head to the Pinwheel Forest, you'll run into Burgh. He will go into the forest. When you enter, you will see him again. Approach him and he will explain that there are two paths: west and north. He takes the north path, so you can take the west path. The first trainers you meet are twins ready for a double battle.
Walk south to battle a Plasma Grunt. He has a new Pokemon you haven't seen before, Sandile. It has Intimidate, which will decrease your opening Pokemon's attack. It also has Torment, which prevents you from using the same attack two turns in a row.
Remember the girl that wanted to trade Pokemon back in Nacrene City? You can catch the Pokemon she wanted in Pinwheel Forest. If you catch a Cottonee in Black or a Petilil in White, just go back to the building east of the Pokemon Center in Nacrene City to trade for the other. It will hold a Cheri Berry, which heals paralysis. Traded Pokemon gain more experience at the end of battles than non-traded Pokemon, so it will gain levels faster than normal.
Walk west to find another Plasma Grunt. Before fighting her, walk south through the grass and west to find a Big Root. The Big Root increases absorbed HP from moves like Mega Drain and Leech Seed by 30 percent, which is great for a lot of Grass Pokemon. Make your way back to the grunt to battle her.
She will give you a Chesto Berry after defeating her. Walk through the hollowed-out log and under the slanted log. Keep following the path and the first green bump on the ground hides another Pokemon Ranger.
The Pokemon Ranger will give you a Pecha Berry after the battle. Down the stairs and through the grass is TM 86, Grass Knot, which is a useful Grass-type move. The amount of damage that Grass Knot does is related to the weight of the Pokemon being attacked. The heavier the attacked Pokemon is, the more damage Grass Knot does. Head back to the Pokemon Ranger and walk through the log. The final Team Plasma member in Pinwheel Forest is waiting for you.
The Plasma Grunt will give you the Dragon Skull that was stolen. Gorm, who is one of the Team Plasma sages, will appear. He is followed by Lenora and Burgh. Gorm and the grunt will vanish, leaving the three of you to talk briefly. You give Lenora the skull and she gives you a Moon Stone. The Moon Stone can evolve certain Pokemon like Munna. Walk east after Lenora leaves to find a Miracle Seed. Hop down the ledge and you will end up back at the road that leads to the north and east exits. You can walk down south to battle some more trainers. A School Girl who is staring at the trees will battle if you talk to her.
That leads you back to the beginning of the forest. You can walk outside of the forest where you entered and talk to the nurse to heal up before continuing. Now you can walk all the way north on the road through the forest to Skyarrow Bridge.