How to install custom rom in my samsung galaxy s3 i9300?

[AC Question]

i wanted to install android 5.1 in my samsung galaxy s3(jelly bean).......so i rooted it and install twrp manager.The rooting process was successful....after that i goes to flashing process using twrp...but something goes wrong and my phone is dead now.when i tried to restart my phone it only shows the samsung logo...so pls help me....i want my phone back as early as possible....