This is not the appropriate section for this question. Post this in the Rs2 development section.
You'll also achieve more success in getting an answer if you go into a bit more detail. What is a custom? Custom item? Custom npc? Custom minigame etc. Just saying custom is a very vague description.
Also, if you were unable to see the large section titled Rs2 development section, you will not have any success in developing a private server.