There are 3 main ways:
For small engines, pull start (like a car)
Belt Driven Start, its just a belt between the
engine and starter (Like Thorjet Fans)
And Prode, this is the easiest method, its just a
probe that connected to the saft of the fan
while the other side is solts into the start.
http://bvmjets.com/df_power/starterprobe.htm