Coil antenna for 100MHz FM transmitter
Take a look at this picture that I have amended: -



Well, it's the antenna.


  I need a coil antenna for a small car module which takes its input
  from my cellphone and transmits it at 100MHz. I have thus far learned
  that I need 8 windings of 0.25 inch inner diameter. I presume I just
  feed the antenna output into this coil, and the other end of the coil
  to GND. Is this correct?


No, that coil is not the antenna - the blue wire is the antenna and I reckon it should be about 75 cm long optimally for the 100 MHz band. This is what the coil does (see L1): -



L1 is part of a colpitts oscillator that is frequency modulated by amplifier Q1.

However, this design is totally illegal (as presented in the guide you linked to) so I can't encourage you to build it at all.