The test is whether you can write down the rule completely. If the honest instruction contains it depends, and you cannot specify what it depends on, the task is not ready to be automated. Attempting it anyway produces a system that handles the common case and fails silently on everything else, which is worse than doing it manually.
Appointment reminders are the highest return automation for most businesses that book time, because no shows are a direct revenue loss and reminders reliably reduce them. That is a rule with no judgement in it, the failure mode is harmless, and the effect is measurable within weeks.
Invoicing and payment chasing is the second, and it is the one owners most often keep manual out of discomfort. Invoices generated on completion, reminders sent on a schedule before and after the due date, and receipts issued automatically. Most late payment is administrative rather than deliberate, and a scheduled reminder does not feel like an accusation the way a personal message does.
Moving data between systems is the third and the least glamorous. Anything you retype from one place into another is a candidate, because retyping is where errors enter and it produces nothing. A form submission that creates a customer record, or a payment that appears in your accounting, removes a task and an error source together.
Start with something where failure is visible rather than silent, which is the safety principle that matters. An automation that stops working and produces nothing is noticed. One that produces subtly wrong output continues for months, and the cleanup exceeds everything it saved.
Do the task manually enough times to understand it before automating, since automating a process you have not stabilised means encoding whatever was wrong with it. The version that emerges after twenty repetitions is usually different from the one you would have built at the start.
Keep anything customer facing personal for longer than feels efficient, because that is where automation is most detectable and least welcome. An automated thank you, follow up, or apology is recognisable and it removes the entire value of the gesture, which was that somebody took a minute.
Check them periodically rather than trusting them, since automations break quietly when a tool updates or a connection expires. A monthly glance confirming that things which should have fired actually did takes minutes and prevents discovering a two month gap.
Then resist automating in place of deciding, which is the failure underneath most disappointing automation projects. A process nobody has defined does not become better by running faster, and the tooling frequently gets bought to avoid the harder question of whether the process should exist.
Document the manual version before automating it, since that document is what lets you diagnose the automation when it fails and what lets somebody else operate it if the tool disappears.
Leave a way to intervene, because every automation eventually encounters something it should not have handled. A process with no manual override produces a wrong outcome at speed, which is worse than no automation at all.
Start with one rather than several, since each automation introduces a dependency and a failure mode, and building three simultaneously means you cannot tell which one broke when something goes wrong.
Review them annually against whether they still match the process, since automations built around a workflow that has since changed continue running faithfully and produce the wrong result quietly.