1) Contact manufacturer and ask them.
2) Powerwasher. Read instructions.
3) Hose, bucket, soapy water, brush, lots of time.
How old is the siding? What is the condition - just dirty
or possibly faded? The manufacturer may suggest
plasticizers or wax-like coatings after the washing
to restore color and sheen, or to prolong the life of
Please provide us with some more details about the
condition of the siding and what you hope to achieve
and we can provide many suggestion, and a few of
the suggestions may even be useful. I clean my
siding with a garden hose about 3 or 4 times a year,
but my requirements are probably extremely different