How do i Rank Skills

In the lost grove. It's the place you land in after sweet Blossom orchard and astryl's tranquillity

further information:
Skills only gain exp in the lost grove (and the maws of madness which will almost undoubtedly be available during fallfest in a few weeks)

attack rank exp is gained both for every use of a move (for the specific move used) and upon victory (for all moves regardless of use in the fight)

Sraw has a guide on rank exp here