Most have a working wallet first.
For folks that have wallet working already do this
Rename your current wallet in system app to GoogleWallet.apk.back
Drop the new one in system/app
For those that have never had a working wallet on there phone do this
Once you get it working and want the new one, do part 1
Read instructions very carefully and download the 2 files before starting
YOU ASSUME AND TAKE ALL RISK
1 . DO A NANDROID, reboot and turn nfc on
2. Flash part 1
3. You will get a few nfc crashes and some slow screens, just keep hitting ok.
4. Open Google wallet and wait for it to do its thing. Again keep clicking ok on the nfc crashes. When Google wallet is done you will either get a wallet fc or a screen saying you don't have network connection.
5. At this point navigate to system/app and DELETE the wallet apk
6 . Navigate to system, open the build prop in text editor and change these 3 lines to exactly:
This will automatically create a backup of the original buildprop
7 . reboot
8. Download Google wallet from the market
9. Open wallet and let it do its thing. Wait for the fc or network error, click through any nfc fc.
For some of you, it may just work at this point.
You're done then
10. Once you get the wallet fc or network error. Navigate to data/app and DELETE the Google wallet apk.
Reboot into recovery and flash part 2
11. Open Google wallet and let it set up. It may take some time.
12. Once your all set up, navigate to system and delete the buildprop.apk and rename the buildprop.bak to buildprop.apk. If you don't see the bak one edit the existing one back to
For ATT users only
Any other carrier, I have no clue. Be smart and keep a copy of the original someplace safe