Every refund starts from the original transaction, not a separate form. Find the payment in your Activity page, open it, and PayPal gives you the option to refund it either in full or for a reduced amount. You can issue more than one partial refund against the same payment later, as long as the total refunded never exceeds what was originally paid.
The 180-day window matters because PayPal won't let you refund a payment through its own tools once that window closes, even if you still have the money sitting in your balance. Past that point, sending money back to a customer means a separate, ordinary payment, not a refund tied to the original transaction.
Timing after that depends on how the customer paid. A refund to a PayPal balance shows up almost instantly, since it's just PayPal moving its own ledger entries around. A refund that has to land back on a card or bank account goes through that card network or bank's own processing, which is why a customer can see the refund confirmed on PayPal's side days before it actually posts to their statement.
How much of the original fee you get back has changed more than once over the years and varies by region and payment type, so check your account's current fee schedule in PayPal's own documentation before promising a customer, or yourself, an exact number.