This will differ depending on what you want to add but for the purpose of this tutorial I am adding a weapon. When adding items it is always best to base it off another item so you should copy another items entry and edit it accordingly.
Here is my entry for a ghurka knife
["ghurka_knife", "Ghurka Knife", [("GhurkaKnife",0)], itp_type_one_handed_wpn|itp_merchandise|itp_primary|itp_secondary|itp_no_parry, itc_dagger, 13 , weight(1.75)|difficulty(0)|spd_rtng(130) | weapon_length(60)|swing_damage(30 , cut) | thrust_damage(40 , pierce),imodbits_none ],
Below are image's showing you what is what
After you have done this for your item then it should be in the game the next time you load it up, for an easy way to see your item ingame use the troop editor (Found on taleworlds forum) and give yourself the item.
Here is the link to the troop editor as it's very useful when you want to test out your items quickly.
If there was anything that I did wrong or did not cover then please tell me so and then I'll edit it, if you need any help with mount and blade then just give me a hollar and I'll do my best to help you out.
Best Regards xPearse