First you need to let the wallet synchronize with the blockchain. On first start that will take up to 15-20 minutes probably. After that encrypt it with a long password (you find this under Settings menu). After that you restart and go to the Help menu and open the Debug window. In it go to the Console tab and in the text field at the bottom write:
generate 100 999999999
then hit enter. Now you're mining.
If you press arrow up, you'll the above text appear again; hit enter again. Repeat this process for about 50 times and then just let it do it's thing and only get back from time to time to do the arrow up + enter thing again.
For more details or visual aid take a look at: