First get on the website :
http://www.drjones.dk/customkeys/generator.php
Second, this link will bring you to the generator section.. The page should look with something like this. It's the one by one hero custom keys.. Longer and harder
[spoiler=]

You can do one-by-one hero there. Like only some hero bother you cause they use for exemple the 't' key and t hold you so you don't cast... EXEMPLE. Now look at the picture above and check, there's 'here/creep' in green. This is where you're at. you can click the shop, patterns etc.
Patterns is the next (all get same hotkey/easy way):
[spoiler=]

There you can make for exemple : QWER as Skill 1 = Q etc... Don't do this if you customised hero. It's one or the other.
As you can see, you can also click the shop patterns or do it 'one-by-one'
[spoiler=]

Extension (Window only) :
***Download*** this before or this will not work. All user of Window vista / 7/ 8 should right-click and execute as administrator. More information at the end. Now, you can see on the picture below that I clicked the highlighted part. Extension (win only). It will bring you to something like this :
[spoiler=]

In this section, there's a lot you can do! First, for item, you can add a keymodifier = for exemple (hold CTRL + Q = first top left item)... FOR EXEMPLE. You can always put NONE... If you put none, you must be sure that these hotkey are not the same as an other hero. For exemple, invoker wich got multiple spell.
Invoker pre-selected hotkey cast :
You can click on voker casting. This will cancel all default hotkeys of spell. Like meteor is D sunstrike is T etc... Now it will be the letter that will be put in these box. I repeat, in-these-box. Wich mean if meteor swap of side... It's not the same hotkey. I repeat, if the spell change of box, it change the hotkey!
[spoiler=]

You can also put hotkey for active the auto-cast.
[spoiler=]

Don't forget that you're in the extension section and that might not work on Linux or Mac.
When you are done with EVERYTHING. All custom keys are made etc. You must click generate button. The version is ok. Even if there no B or C it will work. Get always latest possible. By clicking generate, you'll get a customkeys.txt files. Copy and paste or simply drag it into your root wc3 file. Then when you open the game. you need to go in option>game> check the box with ''custom keyboard shortcut''. This will make your text file work all the time now.
** ONLY THING THAT DOESN'T ALL THE TIME ** : The extension one. There come the use of the 'downloaded' program. You need to open the downloaded file as admin (if on vista or window 7 / 8). Then go in wc3 and hit F5. A beautiful voice will say custom key active. Other hotkeys for exemple voker one, need to be activated too with F8. Autocast too etc etc. You need to active them 'one by one'. Also, it will maybe bug your keyboard to type. Hit the F9 if happen. (All these F# are default one, you can change them as you like). Remember that you can always add a keymodifier wich = to allow two hotkey to be the same adding one you hold... For exemple ALT + something.
If you got any question, problem, post down here. I tried to be the most clear possible. I think after looking at this and the website FAQ and guide. You'll get no problem by using this tool.
Again, Mac and Linux... Sorry extension will prob not work.. Still you can always make normal hotkey work. Just item and stuff like voker will not.
Comment if usefull, if you feel something is missing. I'll edit my post if need.
Put the customkey file in root then restart wc3 if open.
Thanks. Hope it will help some people. It look maybe 'hard'. But to clarify all this. Custom your patterns or hero by hero. Then the extension one. Generate. Put file in WC3. Allow custom key in option. Turn on the program. Play. Once it's done... All you'll need is to re-open the program as admin when you close/open computer. Rest will be there forever. So take the 20 minute it will. Then you'll never (maybe) touch it again.
P.S : I've tried to go a bit deep in it. Like I said, if you want it simple. Just do patterns. Choose whatever you like : QWER. Then do your item extension and bingo. Generate.
Nutz-
viewtopic.php?f=14&t=11893 <------ For mac item. I don't know if using both work.. Check what worth the best for you!!