k im only a 13 year old bassist,so i mght not be rite on the money,lol
k first, u have to do a slide to ur octave a.k.a 12th fret for the slide
[ Tab from: https://www.guitartabs.cc/tabs/t/taylor_swift/fifteen_btab.html ]
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-------------------------12 10-------------------14----------------12 10--------|
-slide-12----14 14 12 12 -------14 14 12 12 16--------14 14 12 12 -------------|
im pretty sure its correct.if not.ah o well. i tried lol
when u slide to 12 , u start all the way at 0 and make ur way up