Choose the decision first
A useful journal should help answer a question, such as when to repeat a sowing or whether a container needs moving. In a UK or Irish garden, broad weather differences make personal observations valuable. Avoid recording large amounts of information that you never expect to use.
Keep an entry short
Note the crop or project, date, location and action taken. Add one observation about the result or conditions. Photograph labels and unusual problems when that saves time. Keep variety names readable; a picture of several anonymous pots is difficult to interpret a year later.
Separate observation from explanation
Write that a plant wilted after a particular spell of weather before deciding why. Record what you checked and what you changed. This makes it easier to revisit an early assumption if another cause becomes apparent.
Review at a useful moment
Look back before buying seeds or rearranging beds. Identify one success to repeat and one difficulty to address. Keep the records in a place that survives a phone change or a damp shed. Consistency matters more than a sophisticated notebook or app that becomes too demanding to maintain.