First, you need to determine which coin or algorithm you want to mine. You may search the web for information on cryptocoins and algorithms corresponding to them.
Next, you need to find a mining pool to which the rigs rented by you will be mining. Sometimes you need to create an account on the pool. For other pools you need to install wallet software for the selected coin and obtain an address for receiving the funds and use that account address as your pool mining login. The specific details depends on the pool itself so please read carefully the information on the pool's website. As a result, you need to obtain the following pieces of information concerning the pool:
You will need this information to connect rigs rented by you to the pool so that they contribute shares to your pool account. If you want to use a pool for renting multiple times, you may store its information in My Pools list and later quickly recall its information when renting rigs.
Finally, you choose a rig for renting at Virtual Mining. Select Find a Rig to Rent from the For Buyers menu, choose an algorithm and you will see the list of rigs available for renting. To help with your choice, you may filter the rig list by time you want to rent a rig for, by hashrate or price range and sort it by price. You may choose any rig from the list and rent it by clicking Rent Now button. If a rig is already rented by someone else, you will see the time remaining until it becomes available for renting again.