First of all, when you downgrade be sure get the sdk kit. Then be sure to download the files as article says, NOW here is where I had many unsuccessful attempts.
When you place the downloaded files in the adb file, you are actually placing them in the root directory of platform tools. as sdk kit has a few folders labeled platform tools i dug thru all the folders and placed a copy in each folder.
I could not for the life of me get emulator to work, so I had to abd push thru the computer, I as well tried evilnulls method for both temp and erm on my sons Phone, neither worked.
O.k after you get file placed in the folder(s), as was my case, you call up command prompt.
From there you need to look see exactly what directly you have sdk installed in, as mine was in root drive of windows, I had to use c:/android yada yada to call up the command for sdk.
Getting past that it sailed right along. Though will say I formatted card with phone, then dropped my file in. Used the stock card came with phone. There was my other hang up. The phone would not for the life of a crack baby just born, find the darn img till I let it format, which to be sure it was fresh I formatted a total of 4 times.
At the final end, I will say, was not a solid 2 days, as when it ticked me off, I walked away and got a beer. On second day, I had so much beer, I really didnt care if I toasted the phone or not, as it was insured, and free first time around.
Also I used the automated method for root on 2.2, which I watched a utube video on and it pointed a link to xda and the dev there. Not at home near lappy at moment or would share a few links, but google is a friend.
One last note, I turned fast boot off, it was as well causing issues, I had in total, near 65 attempts at temp root on my evo, on my sons, I will say, sailed right thru it, after all i learned first time around.