In the Netherlands (I live there...), most people tip by just leaving some cash on the table or by rounding up the amount on the bill or credit card slip.
Amounts tend to not be steep either, there's no such thing as the "expected 15% tipping" in the US. Literally rounding up the bill to a nice round number is common. Say your bill is for 46 Euro, make it 50 for example. If it's 23 Euro, make it 25, if it's 74, make it 80. Etc. etc.
Dutch waiters are by law paid at least minimum wage for their age group, and often more, so they certainly don't rely on tips to get their wages up to something one can survive on. Many also have the job not as a primary money maker but are students earning a bit of money on the side.
And don't tip for terrible service, tip what you think the service was worth, not what you're "expected to tip". If you tip heavily for bad service, you remove any incentive the waiters have to provide good service, which hurts the business and the experience of future visitors there.