Your pattern should tell you to knit a specific number of stitches, do a decrease, knit one more stitch and then turn your work. You should be turning your work in the middle of the row rather than at the end.
Your pattern should then tell you to slip one stitch, purl a specific number of stitches, do a decrease, purl one more stitch and turn your work again.
You will now need to slip one stitch, before knitting until you reach the stitch before the gap. If you spread out the stitches on your left needle the gap where you previously turned should be quite easy to see. When you reach the stitch before the gap you'll need to do a decrease to close up the gap, before knitting one more stitch and turning your work.
Much like in the previous step, you now need to slip one stitch before purling until you reach the stitch before the gap. When you reach the stitch before the gap you'll need to do a decrease to close up the gap, before purling one more stitch and turning your work.
Your pattern will probably ask you to repeat the actions taken in steps three and four for a specific number of rows, or until you run out of stitches by reaching the ends of the row.