In the case of SSK, you always slip as if to knit. There is a variation of it where you would slip on knitwise, slip one purlwise, then knit the two slipped stitches together - some say that this way is 'improved' but I've never found it any different so I stick with the usual SSK.
In patterns in general, if you are asked to slip 1 and it doesn't specify which way, I always slip as if to purl.
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