It's been done a million times, I'm sure, but I just wanted to share my method and see if anyone uses it. Just a warning, though this can work for any item, I just want to say that this guide will be focused around weapons.
Method 1
Editing an existing item.
When I use this method, I usually find a weapon that I like the most for the item I'm making using Thottbot. If I'm looking for a more recently added item (like Tier 9 or 10) however, I use Wowhead.
A screenshot of Blood Fury, the weapon I'll use, from Thottbot.
Next, you're gonna want to go in your 'items' database and search for the name of the weapon you're gonna use, in my case Blood Fury.
Duplicate that row, then change the ID number to an unused one. Somewhere in the 80000's is usually smart. Then look through the tables for things you want to change about the item. A description of every row's function is on the wiki here.
Once you finish editing the table, '.server reload items' ingame and give yourself the item.
Try to keep the damage under a million though, you can easily 5 shot ICC Bosses with 750000.
Method 2
Making an item from scratch.
I don't use this method as much, mostly because it's more work, but it's probably more efficient in mounts or consumables.
Before you even start the process, search for the name of the item you want your item to look like in your 'items' database. Look under the 'displayid' column and copy that. You're gonna need that later.
This is pretty simple. Since it's extremely similar to the previous step, all I'm gonna say is that this article is going to be key in the development of your item.
Make a new row, and make the entry ID something in the 80000s, that usually works well. Fill out the rows using the table above and use the information you copied earlier in the 'displayid' column.
Once you're done with the row, save changes, use the '.server reload items' command ingame and give yourself the item.
Try to keep the damage under a million though, you can easily 5 shot ICC Bosses with 750000.
Based on Justicebringer.
I hope this guide helps somebody, I kinda worked on it. :P