Most people take negative exercises for granted. The best time to do negative exercises is towards the end of your workout when your muscles are getting tired and can't do a complete set of repetitions. By focusing on the negative and slowly lowering the weight it will stimulate additional growth. Do a few repetitions until complete failure. By reaching complete failure (where you can't lift any more of the weight at all) it will stimulate additional muscle growth.
You can use machines to achieve the negatives or use a partner or spotter. I would recommend a partner or spotter if you are using free weights because eventually, when you do these exercises, you will reach a point of complete failure and you don't want to hurt yourself.
Negative workouts are a great way to stimulate muscle growth and can be done with multiple exercises. Some of the more popular exercises are for bicep curls, chest and shoulder workouts. You can use them for other exercises too. It's a good idea to include negative workouts to help gain strength.