1. Start by putting your crochethook through the Front loop of the first stitch, DO NOT pull the yarn through the loop, instead......
2...... Put your hook through the Front loop of the second stitch. This could take some practice, twist your hook to slide it through.
3. Now you have the two front loops on your hook and the loop that was already there, take up your yarn and pull through the two front loops at the same time.
4.Now you can finish your stitch as normal. You have now 2 loops on your hook, take up your yarn and pull through both of them. Now you are finished. You shouldn´t be able to see anything on the outside of your work.
This is a trick you need to practice a bit. In the beginning I couldn´t get the hang of it, but just hang in there, because the result is worth it.
Your work will look better and more professional.