When drilling the holes, be very careful not to crack the pipes. To prevent that, pick the smallest drill you have and work slowly and carefully.
Of course there are times where the model has completely wrong exhausts or ones that are badly made. In this case it is best to completely replace them.
There are a few ways to do that. One is to take ready-made exhausts from another model or a hobby shop. Another is to make them yourself.
One option is to use Styrene rods as they are offered in various sizes, so it is easy to find the ones that exactly match what you’re looking for. Then paint them in the appropriate color.
Another possibility is to use small metal pipes. Small antennas from old phones and radios will work like a charm. You just have to be careful when you cut them down to size as they are soft and can squash easily.
Some cars feature more intricate exhausts like a smaller pipe inside the bigger one or cut at an angle or even not round but in a different shape. These are usually best achieved with soft metal rods, which are easy to bend and shape accordingly.
After you’ve done with the manufacturing process it will be time to mount the finished work on the model. It’s best to cut the old pipes put preserve the muffler and attach the new ones to it with superglue.