Using a double needle is super easy and makes a nicely finished seam! I use my double needle to turn long sleeves into short. The sleeves on this shirt were too short but the shirt was still long enough, so here is what I did...
(You can find double needles here or at any sewing supplies store.)First, cut your sleeves to the length you want them to be plus 1-2" seam allowance.
I only folded the seam once, since it is knit and will not fray, so I used a 1" seam allowance. If you want to fold your seam over twice, leave a 2" seam allowance. Pin seam in place.
Now set up your machine. Attach the extra spool holder to your machine, and place an extra matching bobbin on it. Thread both spools through your machine as if they are one thread.
Replace your standard needle with the double needle, and thread one thread through each needle.
Sew using a straight stitch, sew about 3/4" from the fold, just as you would with a single needle.
Don't forget to back stitch at the beginning and end!
That was pretty simple wasn't it? And it looks much more professional than a single needle!
:) Good Luck!