You need somewhere to write one sentence. That's it.
The minimum viable first entry
Write, or say, one sentence about today. Not three. One. "Cold morning, the bus was late, made it to work fine." Stop there. That's a finished entry, not a rough draft of a longer one.
You need
- A notebook, a notes app, or a voice recorder
- Ten seconds, not two minutes: this is the shortest format on this site
- Nothing else. No prompt, no theme, no minimum detail.
Day 1
Pick the moment right before bed, or right after your alarm. Write one sentence about today. If it starts running to two sentences, cut it to one before you stop.
Week 1
Same moment, same rule: one line. Don't reread what you wrote yesterday before writing today's. The five-year-diary version of this format is built to be read back in bulk, later, not day by day.
When you miss a day
Skip it and move on. Writing two lines the next day to cover the gap breaks the format, since the whole point of one-line-a-day is that it's small enough to never feel like it's behind. There is no makeup entry in this method.
A note on the evidence
There's no dedicated research on this format, positive or negative. Full accounting on the method page. The case for trying it is that it's small enough to actually finish, not that a study proved it works.
Try it by voice: Brain Dump: say one sentence before bed and it builds the same year-over-year archive a five-year diary is designed for.
For the full picture, including who else uses this format, see the one-line-a-day method page.