Start from Varrock and you'll find a trees a bit to the southeast of the castle. Then, teleport towards Tree Gnome Stronghold by OSRS GP using either Spirit Tree or Slayer Ring and plant what you can find there. Use Falador to teleport as well as the farm tree patch in this area. Travel to Lumbridge and the farm tree patch there . Then get to Taverley to find another one. Last tree patch is located in a farming guild however you'll need 45 farming or 60% Hosidius favor in order to gain access to it.
Alongside the regular tree paths, which you'll do most of times, you can learn more about farming in the form of planting fruits trees in patches that are specifically designed for them.
To travel around this path quickly you want to unlock Spirit Tree traveling system. First one of those is located just over the Tree Gnome Agility Training Area that allows you to teleport using Spirit trees. When you're done, teleport to your Tree Gnome Village and leave by speaking to an NPC just outside the metal cage. At the entrance, take another run towards the west. Then, you will find east of Catherby near the bottom of the mountain.
The easiest way to reach the area is by taking a teleporter to Camelot and then running to the to the east. Following that, you will find a patch in Brimhaven. The easiest way of getting there is to use teleportation to Ardougne and taking the charter boat. Or, you could place your home in Brimhaven to increase the speed of travel by cheap RS gold more efficient. There's a fruit tree patch in Lletya but this one involves an extended quest with high expectations. All the fruit trees are growing 16 hours so it is usually carried out only once a day.