How to tip $$$ a tour guide

Bring Euros. Some might accept online payments, others won't. The only way to know is to ask.

Remember, in order to "win" your tip, tour guides can't just show up, they have to bring it.

We hired a guide for a walking tour of Athens and gave Euros as a tip. Yes, best to ask what they accept. Enjoy your stay!

What is an appropriate amount or percentage?

What is an appropriate amount or percentage?

Depends on a few things.

If the guide is independent and works for themselves, I would only add a few euros if they were outstanding.

If they are an employee of a tour company, I would tip no more than 3-5 euros per hour. Again, depends on performance.

Depends on the size of the group as well. If the guide is a personal guide to your party only and gives a very good performance, I would tip 5 euros per hour. If you are part of a large group, 3 euros per hour is plenty.

Euros cash. It's done discreetly hand to hand.

Different country, but a common experience: we typically tipped a château guide in France 50 centimes for a tour. But these were typically château guardians, and the 50centimes was their only pay for the tour. The clue comes at the end of the tour when the guide says " don't forget your guide." If they don't say it, we know that a tip is not expected. With twenty people, he'd earn 10€ for the turn around the château.

For a group tour where you are being charged and the guide is salaried or the owner, I'd give no more than 2€ per person for an hour or so; you have an expected set price; a tip is icing.
A private tour with a private guide, no tip is expected. You've negotiated the price and you pay the expected high price. If you really want to tip, it could be welcome or some professionals could be insulted (museum employees).

If you are on a "free" tour, tip 10-15 per person.

I will be doing several day tours with 9-17 other people in Spain and Portugal in December during a cruise.
The guides and drivers are all employees. I might (depending on the situation) slip the guide an envelope with a 5-Euro bill for half day, 10 for whole day, but say it's for their children or grandchildren. Some people feel funny accepting tips until you say it's for the family.

Whatever amount, Euros cash for sure.

I pay always in Euros. We tend to pay a lot more than what is being discussed here. Our private guides in Crete and South of France in 2021 and 2022were outstanding, we wanted to thanked them.

Guided tours I've been on in France and Potsdam, I always give the tip (3-5 Euro depending on the duration) in cash, ie Euro coins, the one and two Euro coins, which I check to have on me prior to the tour.

Remember, in order to "win" your tip, tour guides can't just show up, they have to bring it.

Indeed. Mostly our European tour guides have been amazing but we did have one in Assisi (recommended by RS) who was on her phone several times with her son while she was showing us around. She brought us into a store to “shop” and we felt as though we were expected to purchase something (we didn’t). The store clerk gave her an envelope with cash as we left. It felt like a lazy tour … she took several breaks to rest and didn’t talk much while we rested.. some of the “color” she provided was around dating Italian men vs. Americans. I don’t think we even hit the full three hours she promised. I am sure we tipped her because we were inexperienced travelers at the time and didn’t know what to expect from a guide.