1.
Turn your work, so that the wrong side is facing you.
2.
Looking at the column of stitches below the stitch you would've knit next had you not turned your work, count down the number of rows stated in your pattern to the correct purl bump.
3.
Insert your left needle upwards into the purl bump, then bring this loop onto your right needle - being careful not to twist it.
4.
Turn your work, so that the right side is facing you. You will then usually (unless your pattern states otherwise) need to knit the loop together with the next stitch.