How to download all my apps in google play when I have flashed a new ROM?
You have two options.

Do a factory reset. Go to Settings &gt; Backup &amp; reset &gt; Factory data reset.
When you are asked to log-in your Google account, it will ask you if you want to download the apps you installed before. Usually, your paid apps will be installed (and possibly some of the free apps). You may have to manually install the others.

Or, you can always use Titanium Backup. Make sure to back-up only user apps, not system apps. Then, restore those user apps after you have flashed a new ROM.