Make sure you have a foot with an opening wide enough for both needles to enter without hitting the foot. Replace your regular needle with the double needle. Place the spools of thread side by side on the thread posts. (Hint- if you only have one spool of the desired thread, wind a partial bobbin of that thread to serve as a second spool.)
Holding the threads together, thread your machine normally- as if they were one thread. When you get to the needle, separate the threads and run one thread through each needle. Experiment with stitch length on a scrap before top stitching your garment.