You can check in for free at the airport counter, but this is no longer common in the US, and all airports have automated kiosks and automated baggage belts.
Online check-in starts 24 hours before departure and ends 1 hour before departure. You can print your boarding pass at home or at a machine at the airport.
Mobile check-in is available by downloading the Alaska Airlines app. You do not need to print your boarding pass, just scan the barcode.