When sewing jersey tops my personal preference is to leave the hems raw and let them roll however for a more polished look you can also use a twin needle. It's actually really easy, here's how you do it...
1. First buy yourself a twin needle, I'm using a Schmetz with the needles set 4mm apart. If you're sewing jersey make sure the pack says stretch.
2. Twin needle attachments can be expensive so threading the machine correctly is really important. Thread your bobbin as normal. For the top however you need two spools of thread. Place the left spool so the thread comes off in an anti-clockwise direction and the right spool so it comes off in a clockwise direction.
3. Thread the left spool first keeping the thread to the left where you have the option and finally threading it through the left needle. Thread the right spool keeping the thread to the right where you have the option and finally threading it through the right needle.
Notice below that the right thread is not caught through the thread guard directly above the needles. Of course every machine will look slightly different but leave the right thread free at this point only.
With caution test the threading by turning the machine by hand and then slowly through some fabric. Consult your manual for any additional instructions.
4. Turn up your hem, in my case it's 2.5cm and press.
5. It's a good idea to test your stitch on some scrap fabric first. For example, I find with my machine (my loyal 25 year old clunker) that I need to relax the foot pressure so as to not stretch the fabric as I stitch. Once you're happy, with the right side up, stitch evenly around your hem.
Give it a press and you're all done.