Assume your bass is lined? If not or possible even if it is begin by treating it exactly like a fretted, this will give you a feel for 'where you are' before you catch the slidings bug.
Listen to other players and watch their styles if possible, but getting your own style is key - you tube has many excellent fretless players with beginners lessons.
Players of note:
Mark Egan, Mick Karn, Tony Franklin, Pino Palladino (Gary Numan/Paul Young), Jaco Pastorius