If your organisation has an existing Change Control process,
this will probably be suitable for your project. You may need
to modify some aspects of it to suit the project. For example,
authority levels may need to be adjusted. One project may
allow the Project Manager to approve variations up to $10k.
Another project, with a more experienced Project Manager,
may have a ceiling of $15k.
Approval levels should not just relate to amount of each variation. They should also include a cumulative figure so that big variations do not get approved as a series of small variations. For example for the previous example, you might put a total limit on $30k or even $15k per month.
If you need to start from scratch to put together a process, we have a document that talks about scope management including variations. It also has an example of a scope variation form that can be used.