Thanks Linkat for bringing the topic back around to something useful.
Based on the inital post, I am also wondering if the the OP was also asking How to leave a tip, as well as how much. We forget that if you live somewhere where people tip, we've grown up observing it and it's procedures.
Let's assume we are talking about the U.S., where tipping is the norm. The way to give tips may/will be different in other cultures/countries.
I can think of times the person unfamiliar may be confused as in "why did you give a tip here, but not there?"
If you are in a place where you sit down, someone takes your order, comes back with your food/drinks, you tip. At the end of the meal, you would get the servers attention and say "Check please" The bill for your food in this case is called a check.
Regardless of whether you are paying with a credit card or with cash, I try to always leave the tip in cash. On the credit card receipt there will be a line that says "Tip". I always draw a line through that, and in the Total line below it write in what the total of the bill was, then sign the credit card reciept. The server will come back, take your signed receipt, and then come back again with your copy. Then, as I am leaving, I will take the money I am going to leave as a tip and leave it on the table, ususally holding a corner of it down with a glass, the salt shaker, etc.
Don't worry, no one else is going to come along and take that money. It's understood that is for the server.
The reason for leaving the tip in cash is because if you add it to the credit card, the server will be fully taxed for it. I will put it on a credit card sometimes, but only if I just don't have any cash on me.
When traveling, or for anyone really, it's a good practice to carry around some cash, in small denominations, like $1, $5, to leave or give as tips. I always try to make sure I have at least five $1 bills, and one or two $5 bills on me, that's reserved for tipping when the occassion arises.
If you are paying cash for the meal, when the server brings check to you, if you have the cash that would equal what the meal cost, plus what you will leave as a tip, just give the server all that saying "keep the change" or "the rest is for you" They will say "Thank you" and that's it.
Let's say though that you don't have close to the exact amount you want to leave. Then, you would give the server a $10 or $20 bill and say "Could you please bring me change for this, with 5 singles?" The server understands you are making change to leave a tip.
However, I always make sure I mention to bring singles, because I've had servers come back with change for a $20 with no singles, and this is annoying, and more important, makes me feel like the server is trying to get you to leave more of a tip than you wanted. This in itself is a sign of a bad server.
If they do this, don't feel shy about saying "I need singles".
When leaving a tip, round to the nearest dollar. When in doubt round up. It's really not usual to leave coins as part of a tip.
Next - restaurants that you walk into, order food from a counter, you pay for it and get the food, and leave or go sit at a table and eat.
If it's a fast food restaurant, you don't leave any sort of tip.
If you go order at a counter, get your food, but you see someone who works there is cleaning up the tables after customers leave, it's a nice thing to leave a dollar.
If you order at a counter, but someone brings your food over, and checks on you during your meal to see if you need anything, and cleans your table, leave 10%, or a least a dollar.
Some places have a "tip jar" on the counter by the register. You are free to put in the coins of your change, or a dollar, or nothing. You won't be treated any differently.
It's important to remember that the person serving you at a restaurant is your equal, and treat and talk to them accordingly. Don't speak to them as if they are beneath you, or treat them badly.
Leave your tip based on their performance on how well they took care of you, and don't penalize them for something beyond their control.
It's not their fault if you don't like the taste of your food. If a restaurant is very busy, and you can see they are working hard and doing their best, don't leave them less of a tip because you had to wait.
Keep in mind that you are traveling abroad to have a good time, learn new things, like seeing how others live.
Don't be such a penny pincher that you can't even leave a decent tip at a restaurant.
Personally, I would never leave 10%, unless it was a situation mentioned above.
You'll find differing opinions, but for me, I think if you can't afford to leave a decent tip, you should have just stayed home.
Now, please be aware that food servers in the U.S. have a very low wage, earn most of their money on tips. I know it's not like that in other countries, but that's a totally different topic for a different thread.
I'm talking about how it is right now, today.
There are other situations for tipping. I've just covered when you are going out to eat. But this is good for a start.
OH! If you are having food delivered to you, tip the delivery driver, tip 15% of the bill, or at least a dollar or two if a small order.