Firstly I'll add screenshots in a little bit but this is a pretty simple way to get some rep.
First you have to download quickmacros from Quick Macros - universal macro program. Automation, keyboard, mouse, toolbars, programming .... It's just an easy way to make macros.
Step 1: Create a new macro, file -> create new macro
Step 2: Open up wow and log in and get on your character.
Step 3: Create as macro that does the following.
/script AcceptBattlefieldPort(1,1) (the equivalent of hitting Join Battle)
/script RepopMe() (if you die in the BG, hits release spirit for you)
/script JoinBattlefield(0)
Cast a spell or do an ability (keeps you from being afk)
Step 4: finish the macro and put it on your toolbar and alt-tab from wow
Step 5: click on your new macro and click the red dot to start recording
Step 6: go back to wow, click on your macro, alt-tab back out
Step 7: click Insert on the recording window. This will add the recording to the blank macro you made.
Step 8: Go into the macro window and remove the first command (you switching back to wow), and the last two commands (alt tabbing out, and clicking the insert button), and voila you have a macro which keeps you afk once.
Step 9: Add the number 60 at the end of the macro, any numbers on new lines tells the macro to wait that many seconds. Also add rep 100000 at the top and tab indent everything under it to repeat it.
Step 10: now join your BG of choice and hit the macro, (USE the Pause/Break key on your keyboard to stop the macro)
Step 11: Once the bg finishes, somehow mark w/ a sticky note or a tape or even a water drop where the Leave battleground button is and then click it.
Step 12: Now DONT MOVE, it is important that you don't move and stay in the spot that you are in when you left the BG. Now use the home and end buttons to scroll thru camera modes till you get to one that you want to use.
Step 13: alt tab out and click your macro (or create a new one) and hit the record button again.
Step 14: This is the important step.
Alt tab into the game.
right click the battle master.
click your macro.
click the spot on the screen where the leave battleground button is.
alt tab out of the game
click replace (or insert if you made a new macro.
Step 15: do the same thing you did before and remove the first command and the last two commands and add the rep 100000 at the top and tab indent everything.
Step 16: Add 2's between all the line to have the program wait 2 seconds between commands.
Step 17: Go back into the game and test this macro (i right clicked the macro and hit properties to bind it to a key). Test to make sure that it clicks everything, and that there is enough time between the right clicking of the battlemaster and the clicking of your macro.
I've used this for awhile and my friends have used it as well, and nobody has gotten banned or warned. You can do this for any BG, but you have to repeat the steps.
Notes:
You will sometimes get afk-reported, its inevitable, but it doesnt happen often in AV, and even less in AB or WSG.
I'm guessing you can get some chat mods that block whispers to enhance this.
You have to keep wow in the foreground for this to work, no question about it. This is good if you leave the house or are sleeping and dont need the computer.
The best thing to do is to check on your bot periodically and if you get reported, stop the macro, go do damage to someone and then restart the macro, most of the time people wont report you twice.