The ssk I'm used to is slip each st individually knit-wise onto the right needle and knit the two together on the right needle.
You will knit them together on the right needle.
You will be decreasing and the ssk will slant the other direction, opposite of the k2tog.
I have no idea why they tell you to slip them on to the left needle and then move them back to the right, That seems weird to me.
I would do it the other way as Knit4pie said.