Usually, i go with a serial cable, these days, people have found simpler ways
Start by downloading lede-17.01.5-ar71xx-generic-tl-mr3220-v2-squashfs-factory.bin from the openwrt website (Don’t use the original, it needs trimming 😉 and you can install it after using the post after this one
rename the file you downloaded to mr3220v2_tp_recovery.bin and put it in your tftp client folder
Setup your computer’s LAN settings to the IP address 192.168.0.66
Connect the eithernet cable to the yellow LAN port farthest away from the blue wan port (Not the one next to it, only this port works)
start your TFTP client, pick your eithernet LAN and connect the lan cable
Hold the WPS/Reset button, and while holding it, start the router, then wait 4 seconds and release it
On your screen, you should see your TFTP client sending a file (on tftpd32 you see a download progress bar)
Wait for the router to reboot, now you have hte openwrt firmware, go ahead and read the previous post on going back to factory 😉
Have a great day