The first thing that you want to do, is to get the crowd aware of the fact that the show is being recorded live. At the same time, you want to ask them to turn their cell phones off, since you don't want any external interference to take place during your recording of the show and the applause.
Now once you have convinced everyone to turn off the phone, go ahead and show them the applause card. Inform them, that whenever they see the card, they need to go on and applaud the show and make as much noise as they possibly can.
In order to get this working perfectly, go on and take it for a test run or two. Just use this to get everyone to terms with the applause card and how they have to go about performing in its presence.
Now once the show is going on, whenever you want the audience to cheer on, go ahead and display the cheer card. Go on and hold it in front of them till the technical crew lets you know that they have gotten the results that they were hoping for. Once this happens go on and remove the card.
You may use the card as many times as you feel is required.