How can I create a jamming signal for desired frequency?
The aim is to get SINR of the desired receiver low. Take a high power signal and spread it in the desired bandwidth. For ex: take any bit sequence and spread it in a bandwidth using CDMA  like pseudo noise sequence. The bandwidth in which the sequence is spread will depend upon the chipping rate of the pseudo noise sequences. If the power of the interferer is high compared to the signal power (3db or more) then the intended signal will be degraded. Even if the GPS is using some pseudo noise sequence this will still be enough to cause interference as long as interferer power is high. Very similar to the Near Far Problem in CDMA

PS: Hope its only for study purposes :)