The often cited "POsitive-negative-positive" method is terrible and is used by those with zero feedback skills.
The most important component I have found from working in a company with an open, expected and established culture of consistent feedback, is that objectivity comes from specificity. This leads to a much better outcome and people having less of an issue taking the feedback onboard as it removes emotions and concentrates on the issue and not the person.
My particular approach, learnt from a previous organisation I worked for, and which works brilliantly:
Take time to actually adequetely identify the 'specific' action/habit/issue that needs correction and then find a clear impact on a given stakeholder. I know this sounds obvious, but is often overlooked. A great way to find if you're are specific enough is that it should come down to something you can see or hear. You can't see someone "being too negative", only actions that lead to the subject being percieved so.
Now keeping the clearly identified action/s in mind, you can approach the person with an open mind, ask for permission to give the feedback (you want to set an environment conducive to the process), and then let them know the problem as well as how it is affecting customers/the team/you.
The impact not the negative action is possibly the crucial element here depending on the actual problem.
Of course, it is important to not come across condescending but it will pay off to not tell the person how to fix this behaviour, but let them offer it to you. I have found that many times people will not know what they were doing was wrong and come to the right conclusion themselves.
Work together to find a way to overcome the issue if necessary, and offer to help in the process or to point them in the right direction—for instance if persons problem comes down to time management, and one of your peers happens to have that as a strength, then maybe they could spend time together on a project and that peer could share the way they effectively manage their time.
After the exchange it's also important to make sure that they know everything is 'cool' between you two, so later in the day/week it may be advantageous to strike up conversation on a completely unrelated topic, or just ask how their week is going, even invite them for a drink after work etc.
If done correctly this approach will work literally every single time. It did not fail me once in four years.
The most important component I have found from working in a company with an open, expected and established culture of consistent feedback, is that objectivity comes from specificity. This leads to a much better outcome and people having less of an issue taking the feedback onboard as it removes emotions and concentrates on the issue and not the person.
My particular approach, learnt from a previous organisation I worked for, and which works brilliantly:
Take time to actually adequetely identify the 'specific' action/habit/issue that needs correction and then find a clear impact on a given stakeholder. I know this sounds obvious, but is often overlooked. A great way to find if you're are specific enough is that it should come down to something you can see or hear. You can't see someone "being too negative", only actions that lead to the subject being percieved so.
Now keeping the clearly identified action/s in mind, you can approach the person with an open mind, ask for permission to give the feedback (you want to set an environment conducive to the process), and then let them know the problem as well as how it is affecting customers/the team/you.
The impact not the negative action is possibly the crucial element here depending on the actual problem.
Of course, it is important to not come across condescending but it will pay off to not tell the person how to fix this behaviour, but let them offer it to you. I have found that many times people will not know what they were doing was wrong and come to the right conclusion themselves.
Work together to find a way to overcome the issue if necessary, and offer to help in the process or to point them in the right direction—for instance if persons problem comes down to time management, and one of your peers happens to have that as a strength, then maybe they could spend time together on a project and that peer could share the way they effectively manage their time.
After the exchange it's also important to make sure that they know everything is 'cool' between you two, so later in the day/week it may be advantageous to strike up conversation on a completely unrelated topic, or just ask how their week is going, even invite them for a drink after work etc.
If done correctly this approach will work literally every single time. It did not fail me once in four years.