To add custom stuff you need to know how to use your SQL viewer (hediSQL or navcat). i like to use hediSQL some people like navcat what ever im telling you how to do this with hediSQL although they are pretty much the same.
ok to make custom items the best place to make them is WoW Vendetta - Wage Your War. There you can make wepons, armor, jewlery, ammo, bags, and vendors to sell it all. most of it is pretty straight forward but there are some parts that need explaining.
lets make a wepon. now the first box has the biggest problem the display id. this is really easy once you know how to get it. go to Wowhead: OMG IT'S ALMOST HERE and search items in the same class as the wepon your making. (etc. 2h sword). i chose Thunderfury, Blessed Blade of the Windseeker. chose your items on looks not stats. now to get the display id all i have to do is look to the adress bar (the bar where you type the www.) and its the last four or five numbers there. All you have to do is copy and paste the numbers into the display id box and your good. Entry # is just the number used to spawn it. i.g. additem 299302 or whatever.
most of the other things are self explanitory. just a word of caution. dont give the items stats over 999. past that the server will tend to crash or the item wont work.
there are a few options for item bonuses. the easy version is recomended but ill go into using the indepth method. so lets say your making a ring and you want it to do something cool (never go into this with a super specific thing in mind. i've been disapointed a lot). go to wowhead and look up spells. say you want this ring to heal the targeted party member. well first we have to establish which classes heal and which one does the most healing. I think priest (going for flash heal not HOTs) do the most healing so i go to database->spells->class skills->priest->holy. now i've found a lot of spells. just search "heal" or what ever it is your looking for. now find the spell you want (if you want it to do a lot of healing or if you want it to do little healing).then click on the skill. i chose greater heal rank 7. Just copy paste the numbers into the box and there you go you have a ring that heals you.
vendors really straight forward not going into to much detail. just find the items you want them to sell on wowhead get the id and copy and paste.
For all entry ids vendor ids try to do something like 18,000,000 or 19,000,000 so i doesn't get in the way of anything.