Tongue and groove floorboards - which way up?
It is thick side up, but to use that for flooring it was not milled properly. Yes the top is to be the thicker side but the thin side is too thin to hold up over time and not crack here or there. There should be a relief cut on the bottom face to help prevent cupping and where the nail goes through to keep the splinters raising the board off the subfloor.

Although I would not use it for flooring, and you are stuck with it, do not use an underlayment paper, use a good construction adhesive over the subfloor to hold it over the long haul and blind nail it down, letting the glue be the work horse here.