How often you should treat a mahogany decking depends largely on the weather and the location of this. If you get constant sunshine and a lot of rain or moisture exposed, you should apply a sealer annually. Otherwise, you have to treat it at least every two years.
Use a pressure washer to clean mahogany decking. This should be clear of debris and dirt before treating. Use softest mode pressure washer to not leave marks on cover.
Starting from far corner, spraying a section at a time until you have cleared entire deck.
Let mahogany decking dry, wood must be completely dry before continuing. Let it dry for at least six hours more in humid climates.