Choose an amount you can miss, even in a dull month. On the day you are paid, move it from the account you spend from into a savings account you do not use for groceries and taps. If your employer can split your pay, send that slice straight to savings so it never lands in the spending account.
Spare-change roundups and coin jars can sit beside this. They are a bonus, not the plan. A set transfer does the real work because it does not depend on how many coffees you bought.
Start this week
Schedule the transfer for your next payday, even if the first amount is small. Raise it only after two pay cycles have felt comfortable.