The general practice is to blind nail through the tongue as with conventional strip flooring. Then countersink one or more flat head screws, No. 7 - No. 9 phillips head or dry wall screws at each end of each plank and at intervals along the plank to hold it securely. Cover the screws with wood plugs glued into the holes. Be sure the screws are the right length.
Some manufacturers recommend face nailing in addition to other fastenings.
Another practice sometimes recommended is to leave a slight expansion crack, about the thickness of a putty knife, between planks. Consult manufacturer's installation instructions for details. The existing wood floor can serve as a subfloor. Drive down any raised nails, re-nail loose boards and replace any warped boards that can't be made level. Sweep and clean the floor well, but don't use water.