Brace to set a fence post?

Simple brace to hold a post in place so water and quick setting concrete can be added. Photos or ideas on this? Thanks.
Update: Well in a previous questions someone said look on the bag for instructions on quick setting concrete. The bag does not give details and it says fill the hole 1/3 with water and then add concrete until there is no surface water. The suggestion given below is to add concrete first.
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