In order to start playing with Play Money, simply follow these steps:
TIP: If you are using the computer version, and don't see the Play Now option this means the table is full. In this case, you can wait for a seat by clicking on Join Waiting List on the right-hand panel or look for a similar table by selecting Play at a Similar Table which is located there as well. In order to avoid full tables, you can tick the Hide Full Tables option located below the Stakes section in the Cash Games lobby (under the Cash tab).
You can get up to 3,000 free chips every hour. Once your chips balance goes below 1,000, you can refill in Play Money cash games, tournaments, and in the Cashier (if the 'Play Money' tab is available). Just look for the Collect or Free Chips button.
If you are already seated in a Play Money cash game, you can get more chips by either clicking on the chip tray or on Options at the top of the table, depending on your table theme. Then click on Add Chips or simply right-click on your own seat, which will bring up the rebuy menu.