To install custom ROMs and firmwares on your Samsung Galaxy S3 Mini, you will need to install clockwork Mod recovery, more popularly known as CWM. This will also let you root your Galaxy S3 Mini which I am sure a lot of S3 Mini users would want to do on their phones. Here are the steps.
1) Download this file (unzip it) and ODIN on your computer.
2) Turn off your phone and enter download mode. To do that, press power key + volume down + home key.
3) Press volume up key to continue.
4) Now plug your phone into your computer and open ODIN.
5) Click on PDA button and select the file downloaded in first step.
6) Press start button to flash the CWM.