A rolled hem on any overlocker is formed by using the right hand needle only, and the two loopers to form a narrow three thread seam. You also need to retract the seam width finger as Trial2962 said. This is a little finger that sits on the stitching plate and the loopers form the stitching over it. By retracting it, the looper threads can form a narrow seam and in fact, roll the fabric inside the seam. You need to move the cutting blade as far right as it will go so you are trimming as wide as possible from the needle, turn the tension on the top looper rigth down to 2 so it is very loose and this thread wraps around to the underside. And tighten the lower looper tension to about 7 so it hardly shows and sits right up against the needle thread. Adjust and test until you get the seam looking like this, then turn the stitch length dial (on the right side by flywheel) down to 0.5 to close the seam right up tight. Lastly, if you can source it in the right colour, buy woolly overlock thread and run this through the upper looper only, this thread is fluffy and when not under tension, relaxes and "fills" out so the seam appears like a continuous coverage over the fabric.
You will need to practice a few runs and go slowly on corners, a curve is obviously much easier to serge than a right hand cover so if you can, cut the fabric with curved ends, much easier to get a great finish. When you chain off at the end, you need to unravel the tail threads and pull them inside the seam for a few mm with a needle to get a smooth finish, then trim the tail and seal with a drop of Fray Stop.
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