1- Modify an existing gametype.
2- Look on what others made and make something like it. (Remember to give Credits)
3- Just play around with codes and see what you end up with.
You might ask:
Where do I find gametypes?
Look here: http://www.mpgh.net/forum/323-call-d...lection.html#1
How do I code GSC?
Look Here: http://www.mpgh.net/forum/308-call-d...rst-mod.html#1
Hope it helps