It should be fine as long as the soft racks are spaced enough - so that it is balanced - but I do reccommend you put everything else in the boot except the snowboard and bag - just to keep the roof light and aerodynamic.
It basically works by putting the soft racks directly on your roof (one towards the front and one towards the rear), you then open your doors and loop the straps via the inside of your car - like how a bicycle helmet straps under your chin - then you tighten.
But some disadvanteges to note - make sure your roof is clean - make sure there is no grit or else the rubbing during the drive will damage your paint. I don't think they are designed for high speed, 100km/h should be ok (hence my reason for just having the snowboard and bag on the roof), just don't overtake too much faster, but of course we never go above 100km/h
If you have no one riding with you - I recommend putting the snowboard inside - recline the passenger seat all the way back and attach the seat belt.