Negative feedback is one of the things that makes us feel extremely uncomfortable to give. The strategy we mostly use with this is just avoiding it. Sometimes the topics become to pressing, that we end up taking a step towards approaching the person, to finally give the feedback. Many times we get just what we expected: it becomes a self-fulfilling prophecy.
Being able to handle situations like that ourselves, makes us feel more secure and self-confident about having a successful conversation.
One way to approach difficult situations is to talk about oneself instead of the other person. Instead of saying "You stink." you could say something such as "I have a very sensitive nose which leads me to having headaches easily through odors." In the first version, you put your evaluation and interpretation on top of the other person and make it look like "the truth". You counterpart feels offended and won't be open to cooperating with you anymore. In the second version you place the focus on yourself and share your own thoughts and feelings with the other person.
Be aware, that in English, it might look like you are talking about yourself when you say "I feel that..." but what is very likely is, that you use a You-message with it: "I feel that you stink." Then you don't talk about yourself anymore but about the other person.
Besides using I-message, you can also use the formular of nonviolent communication: obersation - feeling - need - request.
If you follow this formula, the other person is more likely to give you what you want because they can relate to you. Feelings and needs are universal, everyone has them and once they learn that you have negative feelings or needs, the chances that they will cooperate, are very high. If they still don't want to do what you desire, start to look at their needs and feelings: what is it that they need? If you openly address this in a conversation the other person will see that you care about him and also start to care about you.
Here is an example of how to use the four steps:
Your shoes are in front of your door.
I am concerned that the cleaning person might not be able to remove the dirt around them while cleaning.
So my need for cleanliness would not be met.
Do you think there is some way how we could change that?
All these steps might seem very long at first sight and if you have a really hard feedback to give, they will support you in approaching it in a successful way.