Easiest way is probably that shown on The Real Hustle not long ago, just install a false front panel over the front of the ATM, most people won't notice, this has a card reader and a mini spy camera for watching the PIN entry. Clone the card and off you go.

Another one they showed which was even more brazen was where they created a completely fake ATM in a booth on the pavement, when someone put their card in the person inside the booth just cloned it, and when the screen said "enter PIN" the keyboard just recorded what they typed. Then it threw an error and returned the card. Even better is where you have a co-scammer who uses the machine "normally", i.e. they put their card in, type the pin in and it squirts money at them. This gives people queueing confidence that it's the real thing.