You can make up a template using one of your fence boards. Use this board to mark however many boards you need. It would be easier to cut the boards before installing.
If you are doing a downward radius over a certain length This can be done after assembling fence sections.you could make a template out of 1/4 plywood. Use spring clamps to hold in place to mark the sections.
The best tool to use would be a jig saw.
Hope this was helpful