In Germany, service is included in the bill and waitstaff are paid more normal salaries than in the U.S., so tips are smaller, typically between 5 and 10% of the bill. The tip is not left on the table. Instead, when one asks to pay, the waitperson says the amount of the bill, and one then tells him/her a slightly higher amount while giving him/her the money; the difference is the tip. In practice, one often rounds amounts up to the next Euro or the next 50 cents. If a customer forgets this, the waiter will generally rummage through his/her large change purse long enough to give the customer time to remember to name the new amount.
When paying with a credit card, you can ignore this, BUT it’s not always possible to write in a tip on the printout, so you may have to improvise (e.g. name a higher total before the waitperson enters the amount into the credit card machine, or tip with cash).