The glib answer is: very carefully.
You can easily transfer part of a colony to a new plate by using sterile toothpicks or cotton swabs ("q-tips").
It sounds like the protocol is testing to see if the bacterial colonies are resistant or susceptible to erythromycin as well as whatever antibiotic was used on the original plate. Partition the original plate into sections by drawing on the plastic with a marker. Using aseptic technique, carefully pick a colony from the original plate and then streak a small section on the new, partitioned plate. Try not to scratch the agar when streaking.
JoVE has a great protocol and video describing this method. URL: http://www.jove.com/...plating-methods.
The Replica Plating method in the video details this procedure much better than I can describe it with words. This method starts at 11:52 minutes in the video.
The protocol also describes proper aseptic technique and a few other methods to plate bacteria.