Singapore gives you a way to deal with customs before you land, which is the part most travelers do not realize exists.
Pay before you arrive
The Customs@SG web application lets you declare and pay any duty or GST in advance. Having done so, you use the green channel on arrival rather than queueing at the red one.
Otherwise, the channels
On arrival, the red and green channels are how you declare and pay duties and taxes.
You are required to declare goods on arrival, and it is worth knowing in advance what falls under the duty-free concession and the GST import relief rather than working it out at the counter.
Check restrictions before you pack
Familiarize yourself with prohibited and controlled goods before entering Singapore. Some require a permit from the relevant authority, and finding that out on arrival is the expensive way.
Three situations worth knowing about
Bringing something in temporarily. Hand carried personal goods can be brought in without paying duty or GST if you declare them appropriately on the way in.
Investment precious metals. These may be exempt from GST where the qualifying criteria are met.
Pets. Animals entering Singapore need documentation, agency permits, and specific steps completed on arrival. This is a plan-ahead item, not an arrival-day one.