Cast on three stitches or as many stitches as your pattern tells you to cast on. You can use a cast on method of your choice for this - I use the long tail cast on.
Knit as many rows as your pattern states, in my case I needed to knit eight rows.
After knitting the final row you don't need to turn your work, as you're instead going to work along the edge of your knitting to pick up and knit three stitches, or as many stitches as your pattern states. Looking at the garter ridges, insert your left needle into the loop at the edge of a garter ridge and then knit this loop. Work down the side of your work to pick up and knit as many stitches as you need to.
You now need to pick up and knit three stitches (or as many stitches as your pattern states) along the cast on edge.
You should now have a completed garter tab.