how to use salt to sign a message and how is it effective?
There are a couple of reasons why a salted signature would be helpful. It really depends on the particular implementation of a system.

For example if your message is encrypted, then signing with a salt could give you a different signature for the same message each time. This can be very important depending on the situation.

If the salt was actually provided by the client, then it could be used to prevent man-in-the-middle replay attacks. But there may be better ways to achieve this.