install mac os el capital from SSD
I'm struggling a bit with your wording so lets run through a couple standard tasks and you can let me know if I've answered your question or am way off the mark.

--Situation 1.) You have macOS 10.11 (El Capitan) installed on your MacBook's internal HDD. You want to upgrade with an SSD. 


Attach the SSD using the USB cable.
Hold "Option" at boot and choose the internal HDD with macOS 10.11
Install CCC (https://bombich.com/software/download_ccc.php) and use it to clone the internal HDD to the external SSD. (No need to purchase unless you want to continue using as your backup software)
Restart. 
Hold "Option" at boot and choose the external ssd with macOS 10.11 now on it.
If everything works, shutdown and replace the HDD with the SSD (Search youtube for a walkthrough for your specific model)
Hold "Option" at boot and choose the now internal ssd with macOS 10.11. 
OPTIONAL: If step 2, 5, or 6 failed, Hold "CMD + Option + R" at boot to enter internet recovery. Choose disk utility. Run "First Aid" on SSD. If failed a reinstall of macOS is required. (See Additional Task below)


--Situation 2.) You have macOS 10.11 (El Capitan) installed on your External SSD. You want to upgrade the internal HDD with the SSD. I'll assume that there is nothing important on the internal HDD, but if that is incorrect, please be sure to backup your data first.


Attach the SSD using the USB cable.
Hold "Option" at boot and choose the external SSD with macOS 10.11 on it
If everything works, shutdown and replace the HDD with the SSD (Search youtube for a walkthrough for your specific model) 
Hold "Option" at boot and choose the now internal ssd with macOS 10.11. 
OPTIONAL: If step 3 or 4 failed, Hold "CMD + Option + R" at boot to enter internet recovery. Choose disk utility. Run "First Aid" on SSD. If failed a reinstall of macOS is required. (See Additional Task below)


-- Additional Task: Reinstall macOS (10.13 High Sierra will be installed)


Hold "CMD + Option + R" at boot to enter internet recovery. 
Choose disk utility.  
Ensure desired disk/volume is selected on left side.
Click Erase. 
If HDD choose HFS+ / OSX Extended(Journaled) and name it "Macintosh HD"
If SSD choose APFS and name it "Macintosh HD"
Quit disk utility 
Select Install macOS High Sierra
Follow the steps provided to reinstall. Note that it may take far longer than stated if your internet connection is below average.