OP asked:
"I got ElCap installed. I wish to upgrade to HighSierra. I want to be able to go back to ElCap.
How must i set things up for all this?"
I'll offer a way that's guaranteed go "get you back" if you need to.
And make it VERY easy to do so.
Here's what to do:
1. You'll need an EXTERNAL USB drive
2. You'll need CarbonCopyCloner. Get it here:
www.bombich.com
(CCC is FREE to download and use for 30 days)
3. Connect the external drive to your Mac
4. Open disk utility and erase it to "Mac OS extended with journaling enabled, GUID partition format".
5. Now open CCC and use it to create a BOOTABLE CLONE of your internal drive (El Cap) on the external drive.
6. When done, disconnect the external drive and set it aside in a safe place.
Now you can try upgrading to High Sierra.
BUT...
If the upgrade won't work or doesn't please you, it's now EASY to
"get back, get back, get back to where you once belonged", by doing this:
a. Connect the cloned backup
b. BOOT FROM the cloned backup (boot and hold down the option key continuously until startup manager appears -- now select the external drive with El Cap and hit return)
c. Open disk utility and COMPLETELY ERASE the internal drive.
d. Open CCC and "RE-CLONE" the contents of the external backup BACK TO the internal drive.
e. You now have your old install of El Cap "back where it belonged" on the internal drive.
PRINT OUT this message and keep it in a safe place.
Use it as your guide.
If you DON'T do this, you could still "get back", but it WON'T be that "easy".
One other thing:
If you'd like to experiment with High Sierra BEFORE you commit your internal drive to it, get an external drive and use CCC to clone your internal drive to the external drive.
Now, use the High Sierra installer, but DON'T update the internal drive -- install onto the EXTERNAL DRIVE instead.
Now you'll have High Sierra to experiment with, leaving your internal drive at El Cap for the time being...