I really liked the www.fluentinarabic.com method. It is totally online with real people on the other end. I think I learned more that way than with the softwares I bought. Of course I am not an expert but I have tried several possibilities, this one worked for me.
Hope these $0.02 help ;)
To teach Arabic it is obvious that you should know Arabic. If any question occurs while teaching you must be capable enough to tackle that problem. You can start teaching Arabic by selecting number of student s you think you can handle. Than arrange for proper study material according to the syllabus. Make an agenda of what you want to teach.
Start with basic of Arabic, like the letters of the alphabet. You can get books on Arabic how to proceed further. Once you have started teaching, be consistent with what you are doing. Take tests of students regularly in order to see whether your teaching is going good. If you find any student not doing well, pay special attention on him or her. And give them feedback and ideas on how to improve. In this way you can prove to be a good Arabic teacher.