When you join, you do a slip stitch into the first stitch of ... well, into a stitch. The stitch you did the slipstitch into you also do the SC or DC into.
At least that's what I've been doing whenever a pattern told me to "crochet into the same stitch". Sometimes you have to have a close look to see what stitch it is.
The ways of Yarn are inscrutable.
I'm NOT a "miss", I'm a lady!