I'm by no means an expert and have not been playing fretless as long or as exclusively as many other fine players here, but here's my two cents, for what it's worth, based on what has worked for me over the years:
1. Your ears are THE most important aspect of playing fretless, so develop them and make sure you can hear what you're playing, especially if you plan on playing out with a band. Me, I use in-ear monitors, which are not for everyone but give me the perfect isolation and clarity I need to hear every nuance of what I'm playing and play the way I want to play. This is key, as fretless is heavily dependent on the touch. You don't want to have to be digging into every note just to be heard over a wall of sound, especially when so much of the beauty of fretless voicings lies in a lighter touch. If you don't want to use in-ears, at the very least make sure you can hear what you're doing and possibly get yourself a floor wedge or other monitor. Do NOT let some wanking guitarist turned up to 11 deprive you of your need to hear yourself!
2. Ditto on the setup, unless you know what you're doing (and it seems you might). If you're used to doing your own setup/intonation, be aware that it's a bit harder to do on your own even with a Peterson Strobe because even the slightest mis-positioning (for lack of a better non-word) of a finger can alter your ability to intonate accurately. So the typical open string/12th fret intonation check would need to be further checked across the neck. Some people will use a capo or straight edge pressured across the 12th fret to help, but even that's not 100% perfect. But when all is said and done it's your ears that need to do the most work (see point #1). I do my own setups and I actually have my D and G strings intonated slightly flatter past the 12th fret because I found that my technique (or lack thereof, LOL) and arm/finger angle/balls of fingers were naturally overshooting notes sharp even with the aid of lines. (Yeah, I could learn to play the right way, but I'm lazy). Using my ears, I now comfortably hit the notes I want and I can honestly say my intonation is very good. Perhaps not the ideal solution, but it works well for me, and I've even read interviews with some fretless players who claim they've never had their instruments intonated or adjusted. (Mick Karn, for one). They simply use their ears to find the proper notes and manipulate notes to achieve tunefulness (i.e. trills, vibrato, slides, etc). But I wouldn't recommend learning this way, LOL.
3. Understand the character of a fretless. I first took up fretless because I wanted that tone on some studio recordings I was doing. Using my ears and my lined fretless J, I was able to lay down a "terrific" track on my fretless '77 Jazz. My fellow musicians and the producer commended me on my spot-on intonation. In fact, the producer said, "It's so good I can't tell it's a fretless." And therein was the problem with the track.

I understood how to play it in near perfect tune, but I knew nothing about how to summon forth the voices of the fretless. I simply "played" the notes, plucking back closer to the bridge as I usually do on my fretted Js. But this does bring me to another "happy"? discovery that will no doubt cause some controversy:
4. Bad intonation is like nails on a chalkboard to me, and while I will make no excuses for poor playing I will state that bad intonation is at least less perceptible to our ears in certain frequencies as well as where and how you play along the strings. There are also certain basses I find more forgiving than others in this respect (fretless Precisions, for example), if for no other reason than they offer less of a tonal spectrum than others. Take my Jazz bass example above. Plucking back near the bridge: shorter attack, more staccato tones with less blossom to the notes and therefore, less perceptible poor intonation or "mwah" where nasty sharp/flat notes may be hiding if you're not careful. But PLEASE don't misunderstand me: If you're not at least in the ballpark, nothing's gonna help you.
5. If you're new to fretless but have at least played fretted basses in the past, I'd suggest getting a fretless version of what you're familiar with, at least to start since it can help speed up the process, IMO. Certainly not a prerequisite, but it helped me tremendously. Familiarity breeds content. I played fretted Fender Jazz basses almost exclusively for 20 years so when I pursued fretless about 10 years ago I started on a fretless Jazz since I know the ins and outs of Jazzes and they feel like a second glove to me. I now own 5 fretless basses (Wal Mark I, Pedulla Buzz, Precision, Jazz, and Ashbory), and while they're all different in scale, tone, etc. I can play all of them fine without confusion thanks to cutting my teeth on an instrument I already knew my way around.