Make yourself feel good
Before you go and congratulate the person you do not like, take a deep breath and make yourself feel good about his/her achievement. Convince yourself that you would be giving a good gesture by taking part in someone else’s joy.
In order to feel good and relaxed before you congratulate the person you dislike, you can take part in certain activities, such as going for a long walk or work out, listening to music, read a good book etc.
In order to avoid an awkward feeling, practice in front of the mirror to congratulating him/her. You can also imagine the whole situation, which will help you immensely when the situation eventually arises.
Forget your disliking
The ideal thing to do before you congratulate the person you dislike is to forget about his past behaviour. If you keep thinking about your conflicts, you will never be able to congratulate him nicely.
Have a smile on your face and show pleasure
When the moment arrives, have a genuine smile on your face with a clear heart and congratulate the person warmly, even if you dislike his certain behaviours or acts. You do not need to be too formal in your gestures, as it may give a fake impression.