Question [Q] Recovery not sticking - adb help

Hi guys, I would really appreciate some help. I am able to root my gear and transfer the TWRP via odin. But when I reboot into recovery it just goes back to the stock launcher. I read this in the developer thread:


IF YOU FIND THE RECOVERY IS NOT STICKING & YOUR GEAR WIPES WHEN ATTEMPTING TO ACCESS RECOVERY AFTER FLASHING.
YOU MUST ROOT YOUR GEAR PRIOR TO INSTALLING TWRP AGIAN & RUN THE FOLLOWING COMMANDS IN ADB.

*NOTE* YOU WILL HAVE TO ACCEPT A SUPERUSER REQUEST POPUP WHEN EXECUTING "SU" COMMAND.

*NOTE 2* PROBABLY A GOOD IDEA TO RUN THIS PRIOR TO FIRST INSTALLATION, TO SAVE TIME.

Code:
$ adb shell

$ su

# mount -o rw,remount /dev/block/mmcblk0p20 /system

# rm /system/recovery-from-boot.p
IF YOU DELETE THE "RECOVERY-FROM-BOOT.P" FILE YOU MUST RELFASH TWRP RECOVERY.

I have rooted and installed roms on phone before but working in the developer tools is new to me. Can anyone explain exactly how and when to do this during the TWRP recovery install process? Or possibly just point me to a how to or youtube on basic adb commands? I have searched and it looks super complicated. Just need to know how to run these 3 commands.

THANKS!