Beating the bounds

Within The Forest, a Fae can beat the bounds of an area to claim that space as their dominion. They will then be responsible for everything within that boundary who claims allegiance from them. The limits above ground are also respected below ground, so the courts under the mounds can't grow beyond the line used to beat the bounds between the trees.

Small steadings

For small groups of fae, building a fence or hedge, around a small area is enough for that to become their land. As they move about the ground, tending to it, their reinforce their tie to the land. So, no further process is required to claim the ground for these small steadings.
Once they no longer live there, then the forest will gradually overtake the ground again, returning it to how it was before.

Larger steadings

But for larger steadings, for example for one of the courts of faery, a more substantial ceremony is needed. The process should be repeated about every fifty moons, though it can be done more frequently. As the size of a group changes, the line beaten can be changed. It takes more energy to forge a new larger path, as that is forcing the forest back. But the ruler gains more land, and the possible allegiance of more other fae and servants.
Representatives, at least, from each of the groups within the court, will troop behind their chosen leader around the boundary. The leader takes a branch gifted from the trees, for the courts, this will be from the tree representing their court. They'll process anti-clockwise round the area, using the branch to tap the tree trunks every few paces. Their followers clap to strengthen the binding. When they reach back to the starting point, which should be the entry to the clearing opposite the mound entrance, the process is finished.
By beating the bounds with a troop of fae following, the leader is able to gain their allegiance as well as tie themselves to that area of land. This is how the rulers of the fae courts are granted their powers and responsibility for their court. Every time they repeat the process, they maintain their connection and place in the heirarchy.
The larger the steading is, the longer it will take for the forest to reabsorb that area. Gradually the trees will encroach above and below ground. When their branches touch across the area, it will no longer be a separate steading as it will be back to being forest again.

Other creatures

Most other creatures in the forest follow a similar method to claim their areas. Generally they aren't large, so just by inhabiting a space they will claim it from the forest.
Alternatively, some creatures forgo their need for a separate living space and abide by the rules of one of the courts, acting as their servants.

Competing claims

It isn't impossible for a fae to attempt to claim an area already claimed by someone else. It is unlikely, given that most claimed ground is inhabited above ground. But, if it ever happens then the larger the group tends to win, unless a long enough time has passed since the bounds were last beaten.

Tyramona led the way, her ash switch in one hand. Zoe and the others clapped as they made an anti-clockwise circuit of the mound. They wove their way through the trees, and the almost Queen of Starlight beat every third tree that they passed.

— Fae Secrets