This article 1 gives a short and sweet definition of what makes feedback constructive or unconstructive:
In Study 1, 83 undergraduates received either constructive criticism (feedback that was specific, considerate, and did not attribute poor performance to internal causes) or destructive criticism (feedback that violated these basic principles) of their work.
The study focused on providing a specific kind of criticism, and called that constructive, and a type of criticism lacking these characteristics, which was called destructive.
A second article 2 provides a definition focused on teacher-student relationships:
Research suggests that faculty constructive criticism should be immediate (i.e., timely), specific to the level of performance and skill or task, offer useful and varied strategies for skill improvement, and end with the goal of mastery learning
This study 3 did some research into how students experienced constructive feedback and asked them to define constructive criticism. They identified several categories in students definitions:
a) improvement—definition mentions that the feedback is intended to
lead to improvements or to be helpful or beneficial;
b) strengths—definition mentions that feedback identifies correct or wellperformed aspects of the task;
c) weaknesses—definition mentions that feedback highlights incorrect aspects of the task;
d) kind delivery—definition mentions that the feedback is stated in a face-saving manner;
e) harsh delivery—definition mentions that the feedback is stated
with a critical tone;
f) honest/unbiased—definition mentions that feedback seems straightforward, sincere, and lacking “sugar-coating.”
All in all, this research on constructive criticism seems to support the methods suggested on wikihow:
- Begin in a positive way
- Keep your emotions out of it
- Smile and use warm body language
- Watch the tone of your voice
- Avoid negative language, blaming and personal attacks
- Be specific
- Encourage self-critique
- Focus on the behaviour, not the person
- Make your feedback helpful
- Don't say too much at once
- Know when to stop critiquing
- Follow up
Making sure your criticisms begin in a positive way, that you keep your emotions out of it and that you watch your body language and tone of voice are all a part of a kind delivery, and being considerate. Mention the parts of the task this person performed well, have them realise it's not all bad.
Avoiding negative language, being specific and focusing on the behaviour instead of the person will ensure they feel encouraged to actually try and change their behaviour. It also helps in avoiding attributing a poor performance to internal causes: You're not giving the criticism because the person is bad, but because something they did was poorly performed.
Make your feedback helpful and don't say too much at once: this helps making your feedback more specific, instead of leaving the person feeling that everything they did was wrong. Focus on one thing at a time, and know when to stop. Offer useful insights into what this person can improve and how they can do this, to become better at the task they did.
Since learning is an ongoing progress, it's important to follow up on critique you made. You offered someone points to improve, offering a compliment when you notice the improvement is encouraging, no matter how small the improvement is. Focus on the improvements made. If you do this, it's likely to encourage the person on the receiving end of the criticism to be more open to try and improve the next time too.
All of the above is written from the perspective you mentioned in your question:
Answers should assume that any scenario is one where constructive criticism might actually be listened to.
The third study 3 I mentioned before is an interesting read when it comes to this: Students were divided into groups based on whether they were feedback-seekers (people actually looking for feedback) or feedback-avoiders (people that rather didn't get any feedback at all). It turns out that feedback-seekers are actually the kinds of people that will react better to both constructive criticism and negative feedback (you messed this up) as feedback-avoiders.
1: Baron, R. A. (1988). Negative effects of destructive criticism: Impact on conflict, self-efficacy, and task performance. Journal of Applied Psychology, 73(2), 199-207.
2: Cole, D. (2008). Constructive Criticism: The Role of Student-Faculty Interactions on African American and Hispanic Students' Educational Gains. Journal of College Student Development 49(6) 587-605
3: Fong, J. et al (2016). Deconstructing constructive criticism: The nature of academic emotions associated with constructive, positive, and negative feedback. Learning and Individual Differences, 49, 393-399