If you buy pants that are too big, you can consider the following ways to make them smaller:

1. Find a professional tailor: The most reliable way is to give your pants to a professional tailor. They can measure and adjust your pants according to your body shape to make sure they fit.

2. Shrinkage: If the trousers are made of cotton or other natural fibers, you can try to shrink them by washing and drying. Soak the trousers in hot water for a period of time, and then dry them at high temperature. Please note that this method may affect the color and shape of pants, so please use it with caution.

3. Use shrink belts: There are some shrink belts specially used to shrink pants on the market, which can help you reduce the size at the waist or trouser legs. Fix the speed bump at the position where it needs to slow down, and adjust the size as needed.

4. Sew by yourself: If you have sewing skills, you can sew your own pants and reduce the size. This includes narrowing the waist, shortening the legs or trimming the trouser legs. Detailed guidelines on how to make these changes can be found online.

5. Use a belt: If the waist is the main problem of pants being too big, you can use a belt to help them keep the correct position and provide a better fit.

Please note that you should be careful when trying to change your pants, so as not to damage them or make them unable to return to their original state. If you are not sure how to make these changes, you'd better consult a professional tailor or seamstress to ensure the best results.