There are four different ways in which you can check in for Singapore Airlines flights.
Online Check in for confirmed bookings is open between 48 to 2 hours prior to departure of flights.
Check In can also happen at the airports. Local offices of Singapore Airlines will help you in determining the operating hours of check in.
Self Check In using the internet and mobile is available 48 to 2 hours prior to scheduled departure of flights. You will get boarding passes printed or on your mobile.
Between Singapore and 21 other countries, you have the facility of Return Check In. This means that if you are returning within 48 hours, then you do not have to check in twice, but only once.