In Old School, farming is generally trained by planting and harvesting crops.
Across the RuneScape map, there are over 60 farming patches where you can plant seeds.
Each crop has its own growth time, ranging from 20 minutes to 107 hours. Trees tend to have the longest growth times, while flowers are the fastest.
In Old School, crops have several growth phases.
In each growth phase, the crop has a chance of becoming diseased, and if it is not cured by the next growth phase, the crop dies.
Using compost reduces the chance of your crops becoming diseased. The chance of disease is halved each growth phase when using Regular Compost, one fifth for Super Compost and one tenth for Ultra Compost.
With some patches you can pay the gardener to protect your patch from disease, which guarantees that your patch grows.
Once your patch is fully grown, you can Harvest it for experience and produce. With tree patches, you gain thousands of experience when you harvest the patch.
For patches that yield produce, they start with a set number of Harvest Lives. For example, herb patches start with 3 harvest lives, then for every pick or harvest, there's a chance that the life goes down by 1. Once it hits zero, the patch becomes cleared.
The chance that the harvest life goes down is dependent on your Farming level, Magic Secateurs, the Attas plant, and Achievement Diaries.
Using Compost on a patch increases the number of harvest lives your patch has. With herb patches, Regular Compost gives you 4 harvest lives, Super gives 5, and Ultra gives 6. So, this means the minimum number of herbs you can get per herb patch while using Ultra Compost is 6.
Farming experience in oldschool is mostly gained by doing farm runs. These involve running to various farming patches, planting and harvesting crops periodically, every time your patches are complete.
There are other ways to train your farming which I will show in this guide, although crop farming is the most common, particularly farming trees and herbs.