To Know that you're using Google Wallet correctly:
1. they must have their paypass on (look for a green light on the paypass on the top corner), some places turn them off . 2. you need to tell the cashier "credit" at certain places before it will allow your payment to send but not all the time. 3. you need to have your screen unlocked before touching the phone to the terminal, that includes entering any PINs or Passwords to unlock the phone.
1. Walk in
2. before it's your turn, unlock your lock screen.
3. tell the cashier "credit" or select credit
4. place your phone on the paypass terminal it should double beep and pull up wallet. Enter your PIN
5. Wait for them a second. don't let your phone auto lock the screen.
6. Tap the paypass terminal again. You should see "sending" then "payment sent!" If it requires a signature then sign. Then you should see a virtual receipt on the phone that shows the merchant and the amount. (sometimes it will not update info until later)
7. Once you see this screen, the cashier should get the payment and hand you the physical receipt.
you may need to do step 4 before step 3. If you place your phone at the paypass terminal and it doesn't pull up wallet, of course try again. If you can't get the phone to auto start wallet it means the paypass is probably not turned on, the cashier needs to press credit, or something else is wrong.
Hope this helps. Also for anyone with Wallet questions, may want to check this thread out. this has been discussed at length at XDA Developers:
Provisioning of Google Wallet Prepaid Cards Restored - Page 6 - xda-developers
This is the best youtube example I found, she screws up her placement of the phone on the paypass at first but then it gives you a clear idea of what to expect.
Girl using Google Wallet on her Galaxy Nexus for the first time - YouTube