Ok so I've been having the issue where you are attempting to enter a custom recovery, but you keep getting the "recovery is not seandroid enforcing" in the top left corner and the phone just crashes.
The reason for this is because you've probably been following an old guide/tutorial on youtube that was flashing the custom recovery from Android 5.0.
In order to succesfully flash the custom recovery, you will need to begin by upgrading your phone to Android 6.0.1 and then flash the custom recovery from there otherwise it won't load for some reason.
The easiest way to do this is to flash back to the stock rom using ODIN which you can find here:
Firmware archive | Samsung Updates
On your device, head over to settings, about device and it should read Andriod 5.0.
DON'T attempt to root your device, as we'll need to update to Android 6.0.1.
Samsung will let you do this with an unrooted device by going to about device and clicking Software Updates.
The update is around 900MB and will take around 15 minutes to download and another 5 - 10 minutes to install.
If you HAVE rooted your device, you will be presented with a message saying that you have an "unatuhroised operating system" or something along those lines... And will have to update through Samsung Kies which will take you a lot loner (just trust me!)
Once it's flashed, check back in the about this device and it should read Android 6.0.1
From here just flash the correct Cf-Auto for Samsung Galaxy S5 6.0.1 from this link with ODIN:
https://download.chainfire.eu/397/CF-Root/CF-Auto-Root/CF-Auto-Root-klte-kltexx-smg900f.zip
Leave everyhting ticked in ODIN - shoudln't matter what version of ODIN you used or whether or not it's in administrator.
Once this has been done, boot your device and make sure you have SuperSU and are rooted.
From here, you'll be able to flash the custom recovery succesfully without getting the seandroid error.
I've used twrp 3.0.2.2 which you can get from this link:
https://dl.twrp.me/klte/
Download the img.tar and flash with ODIN.
Once flashed, you just have to boot into Recovery Mode by pressing Volume UP + HOME + Power
Hold all three buttons and at this point you may still get the seandroid text in the top left corner and the phone may keep vibrating twice and rebooting itself.
If this happens don't worry, just hold all three buttons whilst the phone reboots and when you see the seandroid message appear in the top left, LET GO of the power button only.
Whilst still holding the Volume UP + HOME buttons you should then be booted into your chosen custom recovery and can drag and drop CM + G Apps from your pc
Hope this helps some people out there :]