Need help with designing a printed PIFA

i am facing problems in designing two PIFA printed antennas with frequencies 900 and 1800 MHz. Is there any possible way for you to help? or point me to the right direction? i am supposed to design it using CST, HFSS or IE3D. Your help will be much appreciated. This is the first time i ever design an antenna.

http://ap-s.ei.tuat.ac.jp/isapx/2000/pdf/POSA2-1.pdf
http://scholar.lib.vt.edu/theses/ava...ricted/CH3.PDF
http://www.qsl.net/va3iul/Antenna/PI..._F_Antenna.pdf

I am also designing a PIFA now. This is what I do.

1. Draw the shape out in IE3D.
2. Simulate.
3. Do parametrics study, ie vary 1 variable and see the effect on S11 frequency response.
4. Tweak the variable to get the desired resonant frequency and bandwidth you wants.