Very easy I did this in about 30 mins. All you need is wire the push button and the relay which should come with it. Also get one of those probes that checks to see if a wire is hot or not. here is the hook up. Since you want it to be done without the key just cut your starter wire and connect it to the white wire.

[RELAY] --(Red wire)----------->Battery 12V (key off)
[RELAY] --(Yellow wire)-------->IGN 12 (key on)
[RELAY] --(White wire)--------->Starter wire
[RELAY] --(Black wire) <------Push starter---->Ground(chassis)