When you knit with doubled yarn, you use two strands. If you only have one skein, you can use the inside strand and the outside strand. Just knit as normal with the two together. There will be two loops for each stitch on the needle, but you'll easily be able to tell what to knit.
When you knit every row, every other row appears as a ridge. When you see 4 ridges (8 rows of knitting) then you move on to the next instruction. They tell you to count ridges in garter because it's easier than counting rows.