So, I'm working on another slip stitch colorwork project, and thought I would share the process with you.
Here's the front and back of the project. As you can see, this technique wouldn't be good for scarves.
Here are the same sides, stretched out, so you can see the structure of the stitches better.
On this project, the main color (dark) is just knit in plain stockinette. I'm not bringing any of the contrasting color (light) into these rows, although you certainly can. (See my avatar for an example of slip stitch colorwork where you're bringing both colors into each row.) I knit two rows of the main color, and then drop that yarn, pick up the contrasting yarn and knit the next two rows (working in the pattern).
To get the contrasting light color to show up, I just knit this color normally. For the areas of dark contrast color, I hold my contrasting color out of the way on the backside of my project, and insert my needle into the next dark stitch as if I was about to purl it. I slide that loop over onto your needle -- and voila! -- I've slipped a stitch!
Now, I've finished one row of the contrasting color, and I'm working on the back side of my project. Since I'm knitting the contrasting light color in garter stitch, I have to treat this project slightly differently than I would if I were kntting in plain stockinette. I knit the light contrasting color stitches, and then I bring my yarn towards me, and if I were going to purl. I then slip the dark color stitches, purlwise, just like I did on the front of the project. Once I've done that, I bring my yarn back to the front of the project (away from me).
If I were doing plain stockinette, I would always have my yarn towards me, because I would be purling, and I wouldn't have to do all this yarn-moving-around.
Does this make sense to anyone? I didn't want to write out an actual pattern, because this one is copyrighted. But hopefully, this gives a good idea of how this technique works.