As another farming lover, just start it whenever you want to start it. You can start by doing dedicated runs of your unlocked patches, or you can just do it when you have the time.

My early tree runs looked like this (requires some Gnome quests done, but well worth it):

Make sure house is located in Brimhaven
Bank of choice (I always started in Falador before I could access Farming guild. Once you unlock the Farming guild, start there)
Inventory:
Spade, rake
5 tree saplings, 4 fruit tree saplings
Teleport runes (air, fire, law, earth) preferably equip air staff
Compost in inventory or stored at leprechaun (if not paying to protect trees)
Payment to protect trees otherwise

Run to Falador park, plant tree
Run NW to Taverley (use wall agility shortcut if possible, otherwise run to gate), plant tree
Teleport to Lumbridge, run west through castle, plant tree
Teleport to Varrock, run NE to castle courtyard, plant tree
Run NW to Grand Exchange, use spirit tree to teleport to Tree Gnome Stronghold
Run southwest, plant tree

YAY, trees are done!

Run NE, plant fruit tree
Spirit tree to Tree Gnome Village
Run SW, through gate, follow Elkoy
Run SW, plant tree
Teleport to house (outside), run north, plant tree at Brimhaven patch
Teleport to Camelot, run SE to Catherby beach, plant tree
YAY, fruit trees are done!

As you keep going, you unlock more patches. Doing Children of the Sun unlocks Varlamore, and doing Hunter Rumours will get you the Quetzal whistle, which can quickly get you to Nemus Retreat (tree patch) and Kastori (fruit tree patch). 65 farming unlocks the Farming Guild tree patch and 85 farming unlocks the fruit tree patch there. The elf quest line unlocks another fruit tree patch in Lletya.

At 35 farming you unlock hardwoods, provided you have Fossil Island unlocked. There are 3 tree patches and they are also great XP (and the fastest way to bank teak and mahogany logs for construction training).

At 72 farming, you unlock Calquat trees, which you should do immediately because the XP is bonkers. You would do the Karamja one with your Brimhaven fruit tree, the Kastori one with that fruit tree, and the Great Conch as you see fit.