laying hardboard over uneven floorboards?

I just bought some 3mm 610mm x 1220mm hardboard to lay
over the
floorboards in one of the bedrooms as they are a bit
uneven. I have
searched the web to find what is the best way to lay them
but there is so
many people saying to do it all different ways. I have
seen lay the smooth
side up and then someone else said lay the rough side up,
then i seen lay
it the same way as the floorboards and then someone else
said lay it
opposite way to the floorboards. As i heard you say about
putting down
hardboard on qvc could you tell how much water do i need
to use on each
board then when i wet the boards do i need to lay them
flat to dry out or
do i need to nail the boards down when they are wet and
also do i put them
rough side up or smooth up as i will be only fitting a
carpet with no
underlay.