Play lined or unlined fretless, play looking at every note or some or none, do vibrato and slides to your heart’s content or don’t, do whatever you like, but most importantly don’t ever be put off playing because you find any of the things people have said above difficult. I play lined fretless, I do vibrato and slides (not too much but I like it so why not?), I look at the notes but am beginning to look less but so what? It doesn’t improve the quality of the songs I play. Play whichever bass you want in the manner you fancy and feel comfortable with, one person’s method or technique is not another’s, and there is no definitive ‘right’ way to play or learn, do your own thing. I’m not having a pop at anyone or their advice, I’m just trying to stress that it’s easy to be put off by an apparent ‘orthodoxy’ but then I would say that, I’m an old punk who was told as a child and young teenager that I would never be able to play an instrument, let alone be in a band, for a multitude of reasons, all of which vanished when punk came along and suddenly I was in bands playing instruments. There is no right or wrong way with something creative, or maybe there is, just don’t ever think you are doing anything wrong if you are playing your instrument, by all means seek advice and try it but never worry about doing stuff your own way.