New user installing RB750gr3

Hello !
Firstable I'm French so sorry for my english :sweat_smile:

I'm a beginner, I get a Mikrotik for 3 days, I watched YouTube video on how installing openwrt on it.. I download the install file ".bin", put in on file to update it, reboot it but nothing happened. If someone can help me, with a simplified Tuto cause i find nothing in french :sweat_smile:
Thks !