Slip, slip, knit, or SSK for short, is a knitting stitch that can easily be learned from knitting books or videos, such as those found on YouTube. The techniques for SSK vary slightly depending on the location of the stitch and the preference of the knitter.
"Knitting for Dummies" suggests different techniques for the knit side and the purl side of the knitting project. From the knit side, the first stitch on the left-hand needle is slipped onto the right-hand needle and repeated for the second stitch. The loose end of the yarn is wrapped around the right-hand needle, and the two slipped stitches are knitted in the usual manner.
From the purl side, the first two stitches on the left-hand needle are slipped onto the right-hand needle. These same two stitches are then slipped back onto the left-hand needle while still facing the same direction. From the left-hand needle, the two stitches are purled together through the loop on the back side.
The blog from the Purl Bee knitting store recommends an alternating technique that slips one stitch from the knit side and the other from the purl side before knitting the two stitches together. Alternating slip stitches creates a less distinct ridge than knitting from the same side. The difference in technique is often a personal, visual preference.