A bit nerdy maybe but we don't know how nerdy you are, so ...
If you have access to recovery boot menu and the tablet is rooted then you should either use your current SD card or prepare a new one in eg Windows 7.
I would do this: Find your Multi Card Reader and plugin the SD. Backup files on the SD that you want to keep! Use PC software like MiniTool Partition Wizard Home Edition and use all space to create a fat32 partition. The card is now clean and ready to be written on.
Make sure you get the right ROM this time eg needrom.com then copy the zip-file to the SD card.
Make sure your Nexus is completely turned off. Then insert the SD into it.
Look at this thread #4 if you don't know the key combo to recovery: http://forum.xda-developers.com/showthread.php?t=2394113
Usually the SD will mount automatically if nothing else is wrong. Then try out the menu: "install zip from sd card" browse to find the file and then execute.
After this process I would select "clear/wipe cache partition" and even "wipe data /factory reset" just to be sure all is cleaned up.
Finally, select the menu "power off" if it's there.
Then you should be good to go. Be patient. The first startup is always slow. If everything looks fine I would then power off once again to be completely sure.
PS: If you only use Mac I cannot help you there but the procedure is very similar ;)